RADIOACTIVITY: 

AN  ELEMENTARY  TREATISE, 

jfrom  tbe  Standpoint  of  tbe  Bieintegration 


BY 

FREDK.    SODDY,   M.A., 
ii 

LECTURER    ON    PHYSICAL    CHEMISTRY    AND     RADIO-ACTIVITY 
IN    THE    UNIVERSITY    OF    GLASGOW. 


WITH    FORTY    ILLUSTRATIONS. 


COPYRIGHT. 


NEW  YORK: 

THE     D.     VAN     NOSTRAND     COMPANY, 
23,  MURRAY  STREET,  AND  27.  WARREN  STREET. 

ENGLAND : 

THE  ELECTRICIAN"  PRINTING  &  PUBLISHING  COMPANY,  LTD. 
SALISBURY  COURT,  FLEET  STREET,  LONDON 

1904. 


Sb4 


"V, 


Printed  and  Published  by 

"THE  ELECTRICIAN"  PRINTING  AND  PUBLISHING  CO.,   LIMITED, 

1,  2  and  3,  Salisbury  Court,  Fleet  Street, 
London,  E.G. 


PREFACE. 


IN  this  book  I  have  attempted  to  give  a  connected 
account  of  the  remarkable  series  of  investigations 
which  have  followed  M.  Becquerel's  discovery,  in 
1896,  of  a  new  property  of  the  element  uranium.  The 
discovery  of  this  new  property  of  self-radiance,  or  "  radio- 
activity," has  proved  to  be  the  beginning  of  a  new  science, 
in  the  development  of  which  physics  and  chemistry 
have  worked  together  in  harmony.  The  pioneer  in  the 
chemical  development  of  the  subject  was  Mme.  Curie, 
who,  by  the  discovery  of  radium,  extended  our  know- 
ledge of  the  new  property  out  of  the  region  of  the 
infinitely  small  effects  in  which  it  had  its  beginning, 
and  demonstrated  it  on  a  scale  that  could  neither  be 
explained  nor  explained  away.  On  the  physical  side, 
the  brilliant  and  elaborate  researches  of  Prof.  Ruther- 
ford, at  first  mainly  with  thorium — an  element  which, 
like  uranium,  is  so  feebly  active  that  it  had  been  studied 
for  a  century  before  its  radio-activity  was  discovered — 
paved  the  way  for  a  complete  and  general  theory  of  the 
cause  and  nature  of  the-  new  property.  According  to  this 
theory  the  elements  exhibiting  radio-activity  are  in  the 
process  of  evolution  into  lighter  and  more  stable  forms, 
and  the  radiations  spontaneously  emitted  are  due  to  the 
incessant  flight,  radially  from  the  substance,  of  a  swarm 


IV.  PREFACE. 

of  light  fragments  of  the  original  atoms,  expelled  in  the 
course  of  their  explosive  disintegration.  This  theory  has 
recently  received  a  direct  experimental  confirmation  by 
the  discovery  of  the  continuous  production  of  the  element 
helium  from  radium.  In  these  advances  physics  and 
chemistry  have  borne  equal  shares,  and  in  the  close 
communion  between  the  two  sciences  throughout  the 
investigations  the  secret  of  the  rapidity  and  definiteness 
of  the  progress  is  to  be  found.  Radio-activity  has 
passed  from  the  position  of  a  descriptive  to  that  of  an 
independent  philosophical  science,  based  upon  principles, 
only  the  germ  of  which  is  to  be  found  in  physics  and 
chemistry  as  they  were  understood  before  its  coming. 

It  has  been  recognised  that  there  is  a  vast  and  hitherto 
almost  unsuspected  store  of  energy  bound  up  in,  and  in 
some  way  associated  with,  the  unit  of  elementary  matter, 
represented  by  the  atom  of  Dalton.  It  is  possible  to 
arrive  at  this  result  by  two  independent  processes  of 
reasoning,  the  purely  physical  and  the  purely  chemical. 
The  paths  are  different  but  the  conclusions  are  identical. 
Since  the  relations  between  energy  and  matter  constitute 
the  ultimate  groundwork  of  every  philosophical  science, 
the  influence  of  these  generalisations  on  allied  branches 
of  knowledge  is  a  matter  of  extreme  interest  at  the 
present  time. 

It  would  seem  that  they  must  effect,  sooner  or  later, 
little  short  of  a  revolution  in  astronomy  and  cosmology. 
They  will  certainly  be  eagerly  received,  for  it  is  only  fair 
to  add  that  they  have  been  long  awaited,  by  the  biologist 
and  geologist.  By  the  extension  of  the  conception  of 
evolution  to  the  inanimate  world,  which  the  study  of 
radio-activity  has  justified,  not  only  has  that  conception 
achieved  universality,  but  the  difficulties  which  hitherto 


PREFACE.  V. 

have  retarded  its  logical  development  in  the  biological 
sciences  have  been  cleared  away. 

Most  sciences  offer  some  evidence  of  the  extent  of 
time  over  which  the  laws  of  Nature  may  be  regarded  as 
having  been  in  continuous  operation  without  external 
interference.  It  has  been  a  reproach  in  the  past  that 
the  conclusions  arrived  at  were  mutually  inconsistent, 
and  its  final  removal  marks  a  not  unimportant  step  in 
the  history  of  science. 

The  object  of  the  book  has  been  to  give  to  students 
and  those  interested  in  the  subject  generally  a  con- 
nected account  of  the  main  arguments  and  chief 
experimental  data  by  which  the  results  have  been 
achieved.  This  task  would  hardly  have  been  possible 
to  me  but  for  the  training  I  have  received  in  the  subject 
from  Prof.  Rutherford.  I  have  had  the  great  advantage 
of  having  witnessed  the  gradual  development  in  the 
laboratory  of  many  of  his  most  intricate  and  difficult 
researches,  and  I  have  endeavoured  to  pass  on  to  the 
reader  something  of  the  clearness  of  aim  directing  these 
researches  as  it  was  interpreted  and  demonstrated  to  me. 
No  attempt  at  completeness  has  been  made,  for  the 
great  wealth  of  detail  in  which  the  subject  abounds, 
while  it  is  the  opportunity  of  the  investigator,  is  probably 
also  the  chief  cause  of  difficulty  experienced  by  the 
student.  It  is  hoped  that  while  this  book  may  serve  as 
an  introduction,  more  complete  treatises,  or  the  original 
communications,  will  be  studied  in  conjunction  with  it. 

FREDK.   SODDY. 

UNIVERSITY  COLLEGE, 

May  $th,  1904.. 


TABLE  OF  CONTENTS. 


CHAPTER  I. 

PAGE. 

RADIATION  PHENOMENA 1 

Advances  of  the  Last  Decade. — The  Undulatory  and  Corpuscular 
Types  of  Kadiation. — The  Work  of  Crookes  and  Lenard  on 
Cathode  Rays.— X-Rays.— The  Focus  Tube.— The  "Density 
Law"  of  Absorption. — Deviation  of  Cathode  Kay  by  Magnetic 
and  Electric  Forces. — Becquerel's  Discovery  of  the  Radio- 
activity of  Uranium. — Methods  of  detecting  the  New  Types  of 
Kadiation. — General  Account  of  a,  /3  and  7-Rays. — Resemblance 
between  the  Effects  of  the  Undulatory  and  Corpuscular  Forms 
of  Radiation. 

CHAPTER  II. 

THE   RADIO-ACTIVE   ELEMENTS — URANIUM,    THORIUM. 
RADIUM,  POLONIUM  AND  ACTINIUM  15 

Uranium  and  Thorium,  the  only  Examples  of  Radio-activity  among 
the  known  Elements. — Abnormal  Radio-activity  of  Pitchblende. 
— Discovery  of  Radium  and  Polonium. — Radio-activity  an 
Atomic  Property  of  Matter. — Radium — Source — Method  of 
Extraction  —  Atomic  Weight  —  Spectrum.  —  Radio-active 
Properties  contrasted  with  ordinary  Material  Properties. — 
Polonium — Work  of  Marckwald. — Actinium — Work  of  Giesel. 
— The  Five  Radio  elements  and  their  Distinguishing  Charac- 
teristics.— Source  of  the  Energy  of  Radio-activity. —  The  Two 
Alternatives. 

CHAPTER  III. 

THE  ELECTRICAL  PROPERTIES  OF  GASES  37 

The  Ions  of  Gaseous  Conduction. — Distinction  between  the  Ions 
and  the  Radio-active  Emanations.— The  Saturation  Current. — 
Equation  of  Current  flowing  through  a  Gas. — Ionic  Velocities. 
— Coefficients  of  Diffusion.— Determination  of  the  Charge 
carried  by  an  Ion. — The  "Atomic  Charge."— C.  T.  R.Wilson's 
Condensation  Experiments. — Determination  of  the  Number  of 


Vlll.  TABLE  OF  CONTENTS 

CHAPTER  III.— (Continued). 

Molecules  in  a  Cubic  Centimetre  of  Hydrogen.—  Radiant  Ions. — 
Their  Power  of  Ionising  Gases. — Striae. — Determination  of  the 
Velocity  and  of  the  Ratio  of  the  Charge  to  the  Mass  of  the 
Kadiant  Ion. — Direct  Determination  of  Velocity. — The  Negative 
Ions  produced  by  Metals  under  the  Action  of  Ultra-violet  Light. 
— The  Mass  of  the  Negative  Ion  or  Corpuscle.  —Positive- 
Bays. — Electrical  Inertia  or  Mass. — Variation  of  Electrical 
Mass  with  the  Velocity  of  the  Corpuscle  up  to  the  Speeds  of 
Light.— The  Electronic  Constitution  of  Matter. 

CHAPTER  IV. 
METHODS  OF  MEASURING  RADIO-ACTIVITY   57 

Electrical,  Photographic  and  Fluorescence  Methods  of  Measure- 
ment.— Different  Fluorescers  behave  differently  to  the  three 
Types  of  Bays. — Electrical  Method  of  Measurement. — Ap- 
paratus of  Eutherford  and  of  Curie.  —The  Use  of  the  Gold-leaf 
Electroscope. — Distinction  between  lonisation  Currents  and 
Leaks  due  to  Defective  Insulation. 

CHAPTER  V. 

THE  a,   /3  AND  y-RAYS 65 

General  Consideration  of  the  a,  [3  and  y-Rays. — 7-Rays. — Relation 
between  7  and  /3-Rays. — Explanation  of  High  Penetrating 
Power  of  y-Ray  on  the  view  that  it  is  an  X-Ray  Pulse. — /3-Rays 
— Photographic  Action — Magnetic  and  Electrostatic  Devia- 
tion— Value  of  e/m  and  v. — Charge  carried  by  the  /3-Ray. — 
Self-electrification  of  Radium. — Kaufmann's  Work  on  /3-Rays 
approaching  the  Velocity  of  Light. — a-Rays. — Curves  of  Pene- 
trating Powers  of  Various  Types  of  a-Rays. — Diminution  of 
Penetrating  Power  with  Distance  Traversed. — Magnetic  and 
Electrostatic  Deviation. — Value  of  elm  and  v. — Charge  carried 
by  a-Rays. — The  Spinthariscope.  — Confirmation  of  the  Corpus- 
cular Theory  of  Electricity. 

CHAPTER  VI. 
URANIUM  X  AND  THORIUM  X    ' 81 

Radio-activity  an  Atomic  Property.— The  First  Idea  Untenable. — 
Crookes'  Uranium  X. — Decay  of  Activity  of  Uranium  X. — 
Hypothesis  of  Radio-active  Induction. — Uranium  X  gives  only 
/3-Rays. — a  Radiation  a  Specific  Property  of  Uranium. — 
Thorium  X.— A  Specific  Type  of  Matter.— Greater  Part  of 
a-Rays,  all  the  /3-Rays  and  all  the  Emanating  Power,  due  to 
Thorium  X. — Decay  and  Recovery  of  Activity  of  Thorium  X 


TABLE  OF  CONTENTS.  ix. 

CHAPTER  VI— (Continued}. 

and  Thorium — Continuous  Reproduction  of  Thorium  X. — 
Radio-active  Change. — Radio-active  Equilibrium. — Radio-active 
Constant  X. — Independence  of  Radio-active  Change  toward  all 
Known  Agencies. — Curves  of  Decay  and  Recovery  of  Activity 
of  Uranium  X  and  Uranium. — Homogeneity  of  the  Elements 
Thorium  and  Uranium. 

CHAPTER  VII. 

THE  RADIO-ACTIVE  EMANATION  OF  THORIUM  99 

The  Apparent  Variability  of  the  Radio-activity  of  Thorium. — Effect 
of  Air-currents. — Uranium  and  Thorium  Contrasted. — The 
Radio-active  Emanation  of  Thorium. — Emanating  Power  Pro- 
portional to  Weight,  Radiating  Power  to  Surface. — The  Radia- 
tion of  the  Emanation  consists  of  only  a-Rays. — Rate  of  Decay 
of  Activity. — The  Emanation  Analogous  to  the  Argon  Family 
of  Elements. — Decay  of  Activity  Unaffected  by  Temperature, 
&c. — Emanating  Power  Persists  in  Absence  of  the  Atmosphere. 
— Emanation  Condensed  by  Liquid  Air. — Produced  by  Radio- 
active Change  of  Thorium  X. — Imparted  Radio-activity. — Due 
to  Matter  Deposited  from  the  Emanation. — Rate  of  Decay. — 
Imparted  Activity  Concentrated  by  an  Electric  Field. — Pro- 
duced by  Radio-active  Change  of  Emanation. 

CHAPTER  VIII. 
THE  THEORY  OF  ATOMIC  DISINTEGRATION   109 

Radiations  remain  Unaltered  in  Character  during  Decay  and 
Recovery. — Relation  between  Radiation  and  Continuous  Pro- 
duction of  New  Matter. — Both  Rays  and  New  Matter  Result 
from  Parent  Matter  by  the  Same  Change. — Radio-activity 
Measures  the  Number  of  Atoms  Changing. — Non-Separable 
Activity. — Rate  of  Decay  a  Measure  of  Rate  of  Change.— XX 
Atoms  Change  per  Second  when  N  are  Present. — Mono- 
molecular  Types  of  Change. — Disintegration.— Conservation  of 
Radio-activity. — Ultimate  Products  of  Radio-active  Change. — 
Prediction  with  regard  to  Helium — Cause  of  the  Initial  Irregu- 
larities in  the  Curves  of  Decay  and  Recovery  of  Thorium  X 
and  Thorium. — Da-emanation  due  to  Alteration  of  Rate  of 
Escape  of  Emanation. — Rate  of  Production  Constant  as 
Theory  requires. — Disturbing  Effects  of  Subsequent  Changes. — 
General  Resume  of  the  Theory  of  Atomic  Disintegration. — 
Latent  Energy  of  Atomic  Structure. — Explosive  Character  of 
the  Individual  Disintegrations. —  Metabolons. —  Infinitesimal 
Quantity  of  the  Transition-forms. — Unstable  Elements. — 
Criteria  of  Identification. — Average  Life. — Reason  of  the 
Stability  of  the  Elements. 


X.  TABLE   OF  CONTENTS. 

CHAPTER  IX. 

THE  RADIO- ACTIVE  PROPERTIES  OF  RADIUM 127 

Intensity  and  Permanency  of  the  Activity  of  Kadium. — Chemical 
Actions  of  the  Kadium  Bays. — Physiological  Actions. — The 
Emanation  of  Eadium. — The  Imparted  Activity  of  Radium.— 
Kate  of  Decay  of  the  Emanation.—  /3-Radiations  Produced  in 
the  Last  Stages  of  the  Disintegrations. — Summary  of  Radio- 
active Changes  of  Radium. — Evidence  of  the  Complexity  of  the 
Changes  giving  rise  to  the  Imparted  Activity. — "  Induced 
Activity  of  Radium  with  Slow  Rate  of  Dissipation." — The 
Actinium  Emanation. — Charge  Carried  by  the  Emanations. — 
Methods  of  Radio-active  Analysis  of  Minerals. 

APPENDIX.— Table  I.,  Course  of  Disintegration  of  Uranium, 
Thorium,  Radium,  Actinium. — Table  II.,  Radio-active  Con- 
stants and  the  Values  of  the  Average  Life  of  the  Unstable 
Elements. 


CHAPTER  X. 

THE  MATERIAL  PROPERTIES  OF  THE  RADIUM  EMANATION 
AND  ITS  TRANSMUTATION  INTO  HELIUM    149 

Material  Properties  of  Radio-active  Matter. — Condensation  of  the 
Emanations  by  Liquid  Air. — Differences  of  Behaviour  between 
the  Thorium  and  Radium  Emanations. — Volatility  of  Matter 
causing  Imparted  Activity. — Diffusion  Coefficient  of  Radium 
Emanation. — Production  of  Helium  from  Radium. — Production 
from  the  Emanation  — Phosphorescent  Spectrum  of  Radium 
Compounds. — Volume  occupied  by  Radium  Emanation. — 
Diminution  on  Keeping. — Possibility  that  the  a-Particle  is  a 
Helium  Atom. — Law  of  the  Conservation  of  Mass  in  reference 
to  Sub-atomic  Changes. 


CHAPTER  XL 

THE  ENERGY  OF  RADIO-ACTIVE  CHANGE  165 

Energy  Evolved  by  Uranium— by  Radium — by  the  Radium 
Emanation. — Energy  Evolved  on  Disintegration  a  million-fold 
greater  than  during  Chemical  Changes. — Average  Life  of  the 
Radium  Atom. — Theoretical  Estimate. — Experimental  Deter- 
mination.— Internal  Energy  of  Radium  Atom. — Probability 
that  all  Atoms  possess  great  Internal  Atomic  Energy. 


TABLE   OF  CONTENTS.  xi. 

CHAPTEK  XII. 

ANTICIPATIONS    171 

Five  Main  Lines  of  Enquiry  at  Present  Indicated. — (1)  The 
Maintenance  of  Radium  and  Polonium  in  Eadio-active 
Minerals.— The  View  that  these  Elements  are  Transition- 
forms. — Evidence  with  regard  to  Polonium. — The  Present 
Stage  of  the  Enquiry  with  regard  to  Eadium. — (2)  The  Nature 
of  the  Atom. — The  Meaning  of  the  Law  of  Eadio-active 
Change. — The  Eequirements  of  Chemistry. — Differences  between 
Individual  Atoms  of  the  same  Element. — Deduction  that  the 
Component  Parts  of  the  Atoms  must  be  in  Violent  Irregular 
Motion. — (3)  The  Law  of  the  Equivalence  of  Electric  Charges. 
—  Its  Applicability  to  Sub-atomic  Change. — Evidence  of  the 
Simultaneous  Production  of  two  Positive  Charges.  —  The 
Problem  of  Chemical  Valency.— (4)  The  Age  of  the  Earth.— 
The  Controversy  between  Physics  and  Biology. — Lord  Kelvin's 
Estimate. — Consideration  of  the  three  Arguments  in  light 
of  Eecent  Knowledge. — A  Great  Extension  of  the  Older 
Estimates  allowable. — Estimate  of  Maximum  Age  of  the  Earth 
based  on  Radio-active  Considerations. — 1010-years  Limit. — 
(5)  The  possibility  of  the  Eeconstruction  of  the  Heavier 
Elements. — The  Difficulty  of  the  Source  of  Available  Energy. 
— The  Applicability  of  the  Second  Law  of  Thermodynamics  to 
Sub-atomic  Changes.— Clerk-Maxwell's  "Sorting  Demon." — 
The  Two  Alternative  Views.— The  Possibility  of  Cyclic  Evolu- 
tion in  Cosmical  Processes. 


CHAPTER  I. 


RADIATION  PHENOMENA. 

Advances  of  the  Last  Decade. — The  Undulatory  and  Corpuscular  Types  of 
Radiation. — The  Work  of  Crookes  and  Lenard  on  Cathode  Rays. — 
X-Rays. — Tlie  Focus  Tube. — The  "Density  Law"  of  Absorption. — 
Deviation  of  Cathode  Ray  by  Magnetic  and  Electric  Forces. — 
BecquereVs  Discovery  of  the  Radio-activity  of  Uranium. — Methods  of 
detecting  the  New  Types  of  Radiation. — General  Account  of  the  a,  /S 
and  y-Rays. — Resemblance  between  the  Effects  of  the  Undulatory 
and  Corpuscular  Forms  of  Radiation. 

The  last  decade,  1894-1904,  will  probably  always  be  con- 
sidered a  remarkable  one  in  the  history  of  scientific  progress 
on  account  of  the  advances  made  in  connection  with  the 
phenomena  of  radiation.  Not  only  has  there  been  a  great 
extension  of  knowledge  with  regard  to  those  types  of  radiation, 
allied  to  light,  which  enter  into  every-day  experience,  and 
which  have  been  the  object  of  enquiry  for  centuries,  but,  in 
addition,  entirely  new  kinds  of  rays  have  been  discovered,  and 
to  account  for  them  new  conceptions  have  arisen,  fresh  fields 
of  research  have  been  opened  up,  and  problems,  before  deemed 
insoluble,  have  been  brought  within  the  range  of  direct  experi- 
mental attack.  The  previously  existing  foundations  upon 
which  the  vast  fabric  of  modern  science  has  been  successfully 
raised  are  being  exchanged,  without  injury  or  alteration  to  the 
structure,  for  others  one  step  deeper  and  more  fundamental. 
The  work  of  transition  has  been  proceeding  quietly  and  simul- 
taneously from  many  sides,  and  in  this  respect  the  last  decade 
is  inseparably  connected  with  those  proceeding ;  but  until  quite 
lately  few,  except  those  actually  engaged  in  the  work,  realised 


RADIO-ACTIVITY. 

the  magnitude  of  the  results  being  obtained  or  the  real  con- 
sequences of  the  conclusions  being  arrived  at.  Most  recently, 
however,  the  advent  of  radium,  and  the  prominence  given  to 
the  almost  daily  discoveries  that  followed,  have  drawn  universal 
attentiQn?ti>ihe  ftewly  explored  regions.  The  chemist's  atom 

1  is  no  longer  the  unit  of  the  sub-division  of  matter,  and  the 
t<t internal: structu re,  of  the  atom  is  now  the  object  of  experi- 

v  mental  study.  The  particular  development  which  forms  the 
subject  of  the  present  book  arose  out  of  a  discovery  in  1896 
by  M.  Henri  Becquerel,  that  certain  kinds  of  matter  have  the 
property  of  emitting  a  new  and  peculiar  type  of  radiation 
continuously  and  spontaneously,  and  this  class  of  bodies  has 
been  termed  "  radio-active."  This  discovery  was,  however, 
directly  connected  with  previous  discoveries  of  Crookes, 
Lenard  and  Rontgen  of  other  new  kinds  of  radiation,  and  it 
will  be  convenient  to  devote  this  opening  chapter  to  a  brief 
review  of  the  present  position  that  has  been  reached  in  these 
allied  subjects,  treating  each  advance  in  the  order  of  its 
discovery,  and  tracing  its  historical  connection  with  those 
preceding.  This  review  can  at  best  be  but  superficial,  but 
some  idea  of  the  modern  conceptions  of  the  nature  of  radiations 
generally  is  essential  to  the  correct  appreciation  of  the  dis- 
coveries in  the  most  recent  field  of  radio-activity. 

The  term  radiation  is  properly  applied  to  indicate  an 
influence  transmitted  radially  from  its  source  to  its  surround- 
ings, and  capable  of  traversing  vacuous  space,  without 
occupying  in  its  transmission  a  period  of  time  great  enough 
to  be  sensible  under  ordinary  circumstances.  It  was  used 
in  this  sense  to  express  the  transmission  of  light  from  distant 
objects  before  any  definite  ideas  as  to  the  mechanism  of  pro- 
pagation had  been  advanced.  Some  of  the  most  fundamental 
ideas  in  science  owe  their  inception  to  the  necessity  of 
accounting  for  radiation  phenomena.  Newton  was  the  first 
to  recognise  the  difficulty  underlying  the  problem,  for  to 
him,  as  to  present-day  philosophers,  action  at  a  distance 
through  space  of  one  body  on  another  is  inconceivable 
unless  some  connection  exists  between  the  two  bodies.  He 
put  forward  the  only  explanation  possible  in  his  day  to 
account  for  the  radiation  of  light,  and  his  corpuscular  theory, 
as  it  is  called,  although  long  since  disproved  so  far  as  light  is 


RADIATION  PHENOMENA.  6 

concerned,  has  a  remarkable  bearing  on  the  discoveries  which 
form  the  subject-matter  of  the  present  book.  Light,  to 
Newton,  was  propagated  by  the  agency  of  minute  material 
particles  or  corpuscles,  which  are  emitted  from  the  radiating 
object  and  travel  in  straight  lines  from  their  source  in  all 
directions  outward  through  space,  at  a  speed  which  is 
practically  infinite.  This  expressed  an  otherwise  incon- 
ceivable process  in  the  terms  of  matter  and  the  motion 
of  matter.  Science  outgrew  as  it  advanced  these  two  New- 
tonian conceptions.  It  came  to  be  realised  that  something 
else  existed  besides  matter  which  was  capable  of  motion.  The 
idea  of  a  universal  ether  filling  all  space  and  constituting  a 
medium  for  the  communication  of  motion  over  interstellar 
distances  was  naturally  only  admitted  after  it  had  been  con- 
clusively proved  that  the  older  ideas  were  insufficient.  The 
crucial  test,  which  showed  that  the  emission  or  corpuscular 
theory  was  untenable,  arose  out  of  the  properties  of  light 
radiation,  and  from  that  time  the  rival  view,  known  as  the 
unclulatory  theory,  which  postulated  the  existence  of  a 
lurniniferous  or  light-carrying  ether,  has  held  the  field. 
According  to  this  view,  light  is  propagated  as  a  succession 
of  waves  in  the  ether,  much  in  the  same  way  as  the  waves  of 
the  sea  are  propagated.  In  the  latter,  the  moving  particles 
of  water  have  an  oscillatory  motion  in  a  vertical  line,  whereas 
the  wave  travels  forward  horizontally.  In  the  transmission  of 
light  the  oscillatory  movement  of  the  ether  is  also  transverse  to 
the  direction  of  motion.  The  velocity  of  propagation  is  con- 
stant for  all  kinds  of  radiation  which  result  from  the  transverse 
vibration  of  the  ether,  but  the  wave-length,  and,  correspond- 
ingly, the  frequency  or  number  of  vibrations  per  second, 
vary  over  a  very  wide  range.  As  the  former  decreases  and 
the  latter  increases  we  pass,  without  any  sudden  break  of  con- 
tinuity, from  the  invisible  heat-radiations  occupying  the  infra- 
red region  of  the  spectrum  through  the  region  of  visible  light 
from  red  to  violet  to  the  extreme  ultra-violet  waves,  so  easily 
absorbed  even  by  air  that  their  investigation  at  last  reaches 
the  limit  of  experimental  possibility. 

During  the  last  century  the  undulatory  theory  received  an 
enormous  extension  at  the  hands,  primarily,  of  Clerk-Maxwell, 
who  put  forward  the  electromagnetic  theory  of  light  which  is 

B2 


4  11 A  DIO-A  CTI VI T  Y. 

based  on  the  discovery  of  Faraday,  of  the  phenomenon  of  electro- 
magnetic induction.  The  original  undulatory  theory  postulated 
only  one  attribute — namely,  motion — to  the  ether.  Physicists 
occupied  themselves  with  investigating  the  laws  which  the 
motions  of  the  ether  obey.  One  school,  the  most  brilliant  of 
whom  was  Lord  Kelvin,  for  a  long  time  attempted  to  explain  the 
motion  on  ordinary  mechanical  conceptions,  in  which  the  ether 
was  given  certain  material  attributes,  such  as  elasticity  and  in- 
compressibility,  and  the  forces  acting  on  it  were  assumed  to  be 
ordinary  mechanical  stresses  and  strains.  To  the  newer  school 
the  discovery  of  Faraday  of  electromagnetic  induction  supplied 
the  key  to  the  nature  of  the  strains  and  stresses  in  the  ether 
which  produce  light  waves.  Clerk-Maxwell  developed  the 
view  that  a  ray  of  light  was  due  to  waves  of  electromagnetic 
induction  set  up  in  the  ether  by  the  transverse  oscillatory 
movement  of  an  electric  charge,  and  this  view  was  confirmed 
by  the  proof  that  the  velocity  of  propagation  of  electro- 
magnetic induction  through  space  is  the  same  as  that  of  light. 
But  it  was  not  until  after  1888  that  the  view  was  generally 
accepted.  In  this  year  Hertz  produced  electromagnetic  waves 
from  the  oscillatory  discharge  of  the  Leyden  jar,  and  showed 
that  these  waves,  although  of  wave-length  in  some  cases 
several  feet  long,  and  hundreds  of  millions  of  times  longer 
than  the  longest  wave-length  of  visible  light,  yet  travel 
at  the  same  velocity  as  light  waves  and  are  subject 
to  the  same  laws  of  reflection,  rerfaction  and  polarisation. 
The  length  of  wave  is  regulated  simply  by  the  form  and 
dimensions  of  the  apparatus  employed.  To  produce 
waves  of  the  length  of  visible  light  we  should 'have  to  use 
single  atoms  or  molecules  of  matter.  So  the  general  position 
has  been  reached  that  light  waves  are  caused  by  the  rapid 
vibration  or  oscillation  of  electric  charges  within  the  atomic 
or  molecular  structure,  the  oscillation  being  continuously 
maintained  at  a  certain  definite  period  corresponding  to  each 
wave-length  emitted.  For  the  application  of  this  idea  to  the 
problems  of  the  spectroscope,  and  the  effect  of  the  magnetic 
field  on  the  lines  of  a  spectrum,  works  on  spectroscopy  must 
be  consulted. 

Thus,  in  1895,  at  the  beginning  of  the  decade  with  which 
we  are  more  nearly  concerned,  all  the  known  radiations,  from 


RADIATION  PHENOMENA.  5 

the  extreme  waves  of  the  ultra-violet  region  of  the  spectrum 
to  the  long  Hertzian  waves  now  employed  in  wireless  telegraphy, 
were  satisfactorily  accounted  for  as  undulations  or  vibrations 
in  the  luminiferous  ether,  essentially  the  same  in  nature,  and 
differing  only  in  their  wave-length  and  frequency.  At  the 
present  day,  ten  years  later,  this  explanation  holds  good 
without  modification  so  far  as  these  types  of  radiation  are 
concerned,  but  there  has  been  a  great  extension  in  our 
knowledge  of  radiations.  It  has  been  recognised  that  the 
undulatory  variety  is  by  no  means  the  only  example  of 
radiation  existing  in  Nature.  The  rays  we  shall  be  most 
concerned  with  are  of  a  totally  different  character,  and 
constitute  the  realisation  of  the  conception  by  which  Xewton, 
in  his  corpuscular  theory  of  radiation,  sought  to  explain  the 
character  of  light.  Thus  the  term  radiation  to-day  applies 
with  equal  propriety  to  two  fundamentally  distinct  phenomena  : 
(1)  The  older-known  class  of  ether  vibrations;  (2)  a  new 
class  recognised  within  the  last  decade  to  be  caused  by  the 
radiant  expulsion  of  corpuscles  or  minute  particles  of  matter, 
projected  through  space  at  exceedingly  high  velocity.  The 
first  example  of  this  latter  class  was  correctly  recognised  by 
Sir  William  Crookes  30  years  ago,  but  his  explanation  was  not 
then  acceptedT^  Crookes  found  that  when  the  electric  discharge 
is  passed  through  a  nearly  vacuous  space,  as  the  degree  of 
exhaustion  is  increased  the  character  of  the  discharge  alters, 
the  so-called  "dark  space  "  around  the  cathode  fills  the  whole 
tube,  and  "  rays "  proceed  from  the  cathode  normally  to  its 
surface  \vhich  travel  in  straight  lines  through  the  tube  and 
cause  strong  phosphorescence  where  their  passage  is  arrested 
by  the  glass  walls  opposite  the  cathode.  These  "  cathode  rays  " 
have  very  remarkable  properties.  If  any  obstacle  is  placed  in 
their  path  it  is  heated  and  may  be  brought  to  incandescence 
if  the  discharge  is  sufficiently  powerful.  Sharply  defined 
shadows  of  the  obstacle  are  projected  on  the  opposite  walls 
of  the  tube,  the  glass  not  phosphorescing  where  it  is  pro- 
tected by  the  obstacle  from  the  impact  of  the  rays.  Small 
lightly-poised  vanes  of  mica  placed  in  the  path  of  the  rays 
are  driven  round  with  great  speed.  But  the  most  remarkable 
property  of  the  rays  is  that  they  are  deviated  by  a  magnet, 
and,  if  the  latter  is  sufficiently  strong,  can  be  made  to  assume 


6  RADIO- ACTIVITY. 

circular  or  helical  paths  around  the  lines  of  force.  Crookes 
gave  as  the  explanation  of  the  phenomenon  that  matter 
existed  in  high  vacua  in  a  fourth  state,  which  he  named 
ultra  gaseous  or  radiant  matter.  He  supposed  the  effects  to  be 
produced  by  the  flight  of  charged  particles  or  atoms  repelled 
from  the  cathode,  and  attaining  in  the  electric  field  very  high 
velocity  and  kinetic  energy./  This  explanation  has  now  been 
substantially  adopted,  but  only  within  the  last  decade.  We 
shall  see  (Chapter  III.)  that  we  are  forced  to  regard  the  particles 
as  smaller  than  any  known  atoms,  and  that  they  would  be 
more  appropriately  known  as  atoms  of  negative  electricity. 
Without,  however,  anticipating  this  conclusion,  it  may  safely 
be  stated  that  the  cathode  ray  constituted  the  first  known 
type  of  a  corpuscular  radiation  caused  by  the  emission  of  small 
material  particles  or,  at  least,  of  particles  which  possess  the 
ordinary  attributes  of  matter.  In  one  sense  it  is  not  a  radia- 
tion, for  it  is  propagated  normally  to  the  surface  of  the  cathode, 
so  that  with  a  flat  cathode  the  rays  are  propagated  as  a  parallel 
beam.  The  phenomenon  could  only  strictly  be  described  as  a 
radiation  when  a  spherical  cathode  is  employed.  This  is  due 
to  the  radiation  being  directed  along  the  lines  of  the  electric 
force  which  produces  the  motion  of  the  charged  radiant  particle. 
New  types  were  soon  to  be  discovered  to  which  this  restriction 
does  not  apply,  and  which  in  every  sense  conform  to  the 
original  Newtonian  conception  of  light. 

But  many  successive  discoveries  had  first  to  be  made, 
and  the  historical  order  in  which  this  result  has  been  achieved 
is  a  very  interesting  chapter  in  the  progress  of  science.  The 
chapter  opens  in  1895,  at  the  beginning  of  the  present  decade, 
and  this  date  may  be  well  said  to  commence  an  era  of  new  and 
remarkably  rapid  advance  in  the  physical  sciences.  In  this 
year  Eontgen  discovered  the  X-rays.  For  over  20  years 
Crookes  tubes  had  been  in  general  use  for  demonstration  pur- 
poses, but  it  was  reserved  for  Rontgen  to  discover,  and  then 
by  an  accident,  that  they  emit  a  new  and  remarkable  kind 
of  radiation  which,  unlike  the  cathode  rays,  are  capable 
of  passing  through  the  walls  of  the  tube  and  so  penetrating 
the  external  space.  The  rays  are  invisible,  but,  like  the 
cathode  rays  inside  the  tube,  their  presence  is  manifested 
by  their  power  to  cause  strong  fluorescence  when  they 


RADIATION  PHENOMENA.  7 

impinge  upon  certain  substances,  of  which  the  platino-cyanide 
of  barium  is  the  most  commonly  used.  In  this  way  they 
are  virtually  made  visible  to  the  eye,  and  they  were  so 
discovered.  Another  property  they  possess  is  the  power  of 
affecting  sensitised  photographic  plates  in  the  same  way 
as  light,  and  a  third  very  remarkable  property  is  their 
power  of  making  the  air,  or  other  gases,  through  which  they 
pass,  and  which,  under  ordinary  circumstances,  are  practically 
perfect  insulators,  capable  of  conveying  limited  quantities  of 
both  positive  and  negative  electricity.  This  process  is  known 
as  ianisationj  and  the  rays  are  said  to  ionise  the  gases — i.e.,  to 
make  them  for  the  time  being  partial  conductors  of  electricity. 
These  three  methods  furnish  the  means  whereby  the  invisible 
X-rays  of  Rontgen  may  be  investigated.  An  explanation  of 
the  manner  of  their  production  from  a  Crookes  tube  was  soon 
forthcoming.  Their  source  was  traced  to  the  obstacles  bom- 
barded by  the  cathode  rays.  If  the  latter  had  an  uninterrupted 
passage  through  the  tube,  the  X-rays  resulted  at  the  glass 
against  which  the  cathode  rays  impinged.  But  it  was  soon 
found  that  far  more  powerful  effects  could  be  obtained  by 
inserting  into  the  path  of  the  cathode  rays  an  obstacle  in  the 
form  of  a  plate  of  one  of  the  heaviest  metals — platinum, 
iridium,  osmium  or  uranium.  Platinum,  on  account  of  its 
other  valuable  qualities,  is  most  frequently  employed.  More- 
over, if  the  cathode  is  made  concave,  the  rays,  being  expelled 
normally  to  its  surface,  converge  to  a  "  focus."  At  this  focus 
the  platinum  plate,  or  "  anti-cathode"  as  it  lias  come  to  be 
called,  is  placed  somewhat  obliquely  to  the  path  of  the  rays. 
This  is  the  disposition  in  the  ordinary  "focus-tube  "  which  was 
first  designed  by  Prof.  Herbert  Jackson,  of  King's  College. 
The  effect  is  represented  in  Fig.  1.  AB  is  the  concave  cathode, 
and  the  dotted  lines  represent  the  cathode  rays  converging  to 
the  focus  F,  where  they  strike  the  anti-cathode.  The  X-rays 
radiate  away  from  the  plane  surface  of  the  anti-cathode  in  all 
directions,  and  cause  the  glass  of  the  tube  to  fluoresce  strongly 
and  uniformly  over  a  hemispherical  area,  CDE,  bounded  by 
the  plane  of  the  anti-cathode.  A  satisfactory  theoretical  expla- 
nation, on  the  electromagnetic  theory,  of  the  nature  of  the 
X-rays  was  soon  put  forward.  When  the  charge  carried  by 
the  cathode-ray  particle  is  suddenly  accelerated  (negatively), 


8  RADIO-ACTIVITY. 

and  its  velocity  reduced  to  zero  almost  instantaneously  by  its 
collision  with  the  dense  material  of  the  anti-cathode,  a  pulse 
of  an  electromagnetic  nature  radiates  from  the  latter,  and 
this  pulse  constitutes  the  X-ray. 

Thus  the  X-rays,  like  light,  are  ether  waves,  and  the  difference 
seems  to  be  that  in  the  former  the  disturbances  are  of  the  nature 
of  sudden  pulses  very  rapidly  dying  away,  whereas  in  light 
there  is  a  regular  succession  of  undulations  of  the  same  kind. 
This,  together  with  their  probably  extremely  short  wave-length, 
would  account  for  the  fact  that  the  X-rays  have  not  been 
reflected  or  refracted  or  polarised,  although  in  their  nature  they 
so  nearly  resemble  light  rays. 


FIG.  1. 

The  X-rays  are  of  a  far  more  penetrating  character  than 
the  cathode  rays,  and  easily  penetrate  the  glass  walls  of  the 
tube.  The  higher  the  degree  of  exhaustion  of  the  tube  the 
greater  the  penetrating  power  of  the  X-rays  produced.  A 
tube  in  which  the  exhaustion  has  not  been  carried  to  the 
extreme  limit  gives  out  an  easily  absorbed  type  of  X-ray, 
and  is  known  as  a  "  soft  "  tube.  In  a  "  hard  "  tube,  on  the 
other  hand,  the  vacuum  is  so  good  that  a  very  great  difference 
of  potential  between  the  electrodes  is  necessary  to  force  the 
discharge  through ;  the  cathode  rays  in  consequence  attain  a 
very  high  velocity,  and  the  X-rays  they  produce  on  impact 
with  the  anti-cathode  a're  of  a  high  penetrating  power. 


RADIATION  PHENOMENA.  9 

The  difference  of  penetrating  power  between  the  X-rays  and 
the  cathode  rays  is  one  of  degree  only,  and  not  of  kind.  The 
cathode  rays  share  to  a  limited  extent  the  property  of  the  X-rays 
of  penetrating  matter  which  is  completely  opaque  to  ordinary 
light.  The  ordinary  ideas  connected  with  the  terms  "  transpa- 
rency "  and  "  opacity  "  are  derived  solely  from  the  behaviour 
of  light  in  every-day  experience,  and  a  type  of  radiation  to 
which  these  ideas  does  not  apply  seems  strange  and  abnormal. 
In  reality  the  behaviour  of  light  is  more  remarkable  than  that 
of  the  X-rays  and  the  cathode  rays.  To  these  new  kinds  of 
radiation  all  matter  is,  as  a  first  approximation,  equally  opaque 
or  equally  transparent  if  equal  weights  are  compared.  For  equal 
thickness  the  absorption  is  roughly  proportional  to  the  density, 
and  independent  of  the  nature  of  the  matter  traversed.  Thus 
Lenard  found  that,  if  windows  of  thin  aluminium  foil  are 
inserted  in  the  walls  of  a  Crookes  tube  in  the  path  of  the 
cathode  rays,  the  latter  are  able  to  penetrate  the  foil,  and  their 
absorption  by  different  kinds  of  matter  could  be  investigated 
outside  the  tube.  He  found  that  bodies  as  different  in  their 
nature  as  air  and  gold  absorbed  the  rays  very  approximately 
in  proportion  to  their  density. 

Compare  with  this  simple  result  the  behaviour  of  light. 
Certain  bodies,  like  glass,  most  liquids  and  gases,  rock  salt, 
quartz,  calcite  and  other  dense  crystals,  hardly  absorb  light  at 
all,  while  other  substances,  especially  the  metals,  are  almost 
completely  opaque  even  in  the  thinnest  layers.  There  has 
been  found  to  be  a  connection  between  the  transparency  of 
matter  and  its  electrical  properties.  With  but  few  exceptions 
the  transparent  bodies  are  the  best  insulators.  The  whole 
range  of  frequencies  should  be  taken  into  account,  and  not 
merely  the  visible  part  of  the  spectrum.  Thus  ebonite,  accord- 
ing to  Prof.  Silvanus  P.  Thompson,  is  very  transparent  to  the 
infra-red  region  of  the  spectrum,  while  Prof.  Wood  has 
recently  described  a  dye,  nitroso  di-methyl  aniline,  which  is 
opaque  to  the  visible  rays,  but  transparent  to  the  extreme 
ultra-violet  rays.  When  a  body  exists  in  two  forms,  one  of 
which  is  transparent  or  translucent  and  the  other  is  opaque — 
for  example,  selenium,  the  double  iodide  of  mercury  and 
copper,  &c. — the  transparent  form  is  always  the  best  insulator. 
These  facts  serve  to  indicate  that  there  probably  exists  a 


1 0  EADIO-ACTIVIT  Y. 

connection  between  the  electrical  properties  of  matter  and  its 
transmission  of  light  and  are  here  introduced  for  the  sake  of  con- 
trast. To  the  new  types  of  radiation  it  does  not  seem  to  make 
much  difference  whether  the  matter  traversed  is  gaseous,  liquid 
or  solid,  insulating  or  conducting,  "  transparent  "  or  "  opaque." 
The  absorption  is  regulated,  at  least  mainly,  by  the  mass 
of  matter  traversed  and  not  by  its  nature.  Thus,  although 
the  cathode  rays  are  considered  to  consist  of  radiant  particles 
and  the  X-rays  of  electromagnetic  pulses,  the  two  types  exhibit 
a  surprisingly  close  resemblance  in  their  properties.  Both 
ionise  gases,  affect  the  photographic  plate,  and  excite  fluor- 
escence, and  both  are  independent  of  the  more  or  less  arbitrary 
considerations  which  apply  to  the  absorption  of  light  by 
matter.  They  are,  however,  distinguished,  first,  by  the  fact 
already  mentioned,  that  the  X-rays  are  propagated  radially 
with  uniform  intensity  in  all  directions  from  their  source,  and 
secondly  by  the  action  of  a  magnetic  field. 

The  path  of  the  cathode  rays  is  deviated  by  a  magnet, 
whereas  the  path  of  the  X-rays  is  not  known  to  suffer 
deviation.  In  this  way  it  is  easy  to  show  that  the  cathode 
rays  are  the  cause  and  the  anti-cathode  the  source  of  the  X-rays, 
for,  if  the  former  are  deviated  by  a  magnet  so  as  to  no  longer 
impinge  on  the  latter,  the  production  of  X-rays  to  a  large 
extent  ceases. 

In  1896,  with  the  awakening  that  followed  the  researches 
of  Lenard  and  Rontgen  to  the  existence  of  new  types  of 
radiations  of  a  character  utterly  different  from  those  of  light, 
came  the  discovery  of  the  property  of  radio-activity.  At  first 
it  was  thought  that  the  fluorescence  of  the  glass  in  a  Crookes 
tube  under  the  impact  of  the  cathode  rays  was  the  cause, 
rather  than,  as  we  now  know,  an  accidental  accompaniment 
of  the  X- radiation.  M.  Poincare  suggested  that  the  production 
of  X-rays  might  be  an  effect  general  to  fluorescence  which  had 
previously  been  overlooked,  in  the  same  way  as  it  had  been 
overlooked  during  the  30  years  in  which  the  Crookes  tube  had 
been  in  use. 

M.  Becquerel,  acting  on  this  idea,  examined  some  fluorescent 
compounds  of  uranium.  His  method  was  to  place  the  bare  salt 
above  a  photographic  plate,  which  was  carefully  wrapped  up  in 
opaque  material,  and  so  protected  completely  from  the  direct 


RADIATION  PHENOMENA.  11 

action  of  light,  and  to  expose  the  salt  to  direct  sunlight, 
so  as  to  cause  it  to  fluoresce.  He  found  that  his  plate  was 
affected  in  these  circumstances,  even  when  a  layer  of  copper 
foil  or  aluminium  was  interposed  between  the  substance  and 
the  plate.  But  M.  Becquerel  also  soon  found  that  the  exposure 
to  sunlight  was  unnecessary,  and  the  same  effect  was  obtained 
in  absolute  darkness,  even  when  compounds  of  uranium  were 
employed  which,  since  their  preparation,  had  never  been 
exposed  to  light.  Moreover,  he  found  he  obtained  the  same 
result  whatever  compound  of  uranium  was  employed,  whether 
it  was  fluorescent  or  not,  and  he  soon  satisfied  himself  that 
he  was  investigating  an  entirely  new  property  of  the  dement 
uranium  which  was  completely  unconnected  with  the  property 
of  fluorescence  (see  Comptes  Rendus,  CXXIL,  1896,  pp.  420, 
501,  559,  609,  762  and  1,086).  This  property  is  now 
generally  known  by  the  special  term  "  Radio-activity,"  and 
its  investigation  has  already  lead  to  some  remarkable  con- 
clusions of  a  very  far-reaching  character.  The  characteristics  of 
the  radiation  from  uranium  are  very  similar  to  those  of  the 
X-rays.  The  penetrability  of  the  rays  for  ordinary  matter  is,  as 
a  first  approximation,  a  function  of  the  density  of  the  matter 
and  not  of  its  nature.  The  rays  ionise  the  air,  or  other  gases 
through  which  they  pass,  and  make  them  for  the  time  being 
limited  conductors  of  electricity.  This  can  be  easily  shown 
by  a  gold-leaf  electroscope,  which  is  one  of  the  oldest  of 
electrical  instruments,  and  has  suddenly  assumed  a  new  impor- 
tance as  a  means  of  detecting  and  measuring  the  ionisation  of 
the  air,  and,  therefore,  the  intensity  of  the  new  kinds  of 
radiation.  It  is  strange  to  reflect  that  it  was  not  for  the  lack 
of  means  of  detection  that  the  property  of  radio-activity 
remained  lor  so  long  unknown. 

Fig.  2  represents  a  simple  form  of  gold-leaf  electroscope,  charged  and 
uncharged.  It  consists  ordinarily  of  two  narrow  gold  leaves  fastened  to 
either  side  of  a  strip  of  metal,  which  is  supported  inside  of  a  glass  vessel 
coated  internally  with  tinfoil,  by  means  of  a  rod  passing  through  a 
stopper  of  some  good  insulator,  such  as  paiaffin,  shellac  or  ebonite.  To 
charge  it  a  piece  of  rubbed  sealing-wax,  or  ebonite,  is  brought  in  contact 
with  the  upper  end  of  the  rod,  and  the  leaves  diverge  owing  to  the 
repulsion  of  similar  electrical  charges.  If  the  insulating  stopper  is  in 
proper  condition,  the  leaves  once  charged  should  remain  so  for  hours,  or 
even  days.  If  a  small  quantity  of  uranium  compound  is  put  inside  the 


12 


RADIO-ACTIVITY. 


vessel  the  leaves  collapse  within  a  few  minutes.  By  making  the  instru- 
ment small  the  effect  may  be  sufficiently  rapid  to  be  followed  with  the 
eye.  The  sensitiveness  of  the  instrument,  and  the  rate  of  collapse  under 
given  conditions,  depends  on  its  capacity,  and  this  is  regulated  mainly  by 
the  area  of  the  leaves  and  their  support  which  receive  the  charge. 

It  may  be  pointed  out  that  the  electroscope  would  retain 
its  charge  indefinitely  in  vacuo,  even  in  the  presence  of  uranium, 
or  when  acted  upon  by  the  X-rays — i.e.,  the  gas  present  plays 
a  direct  part  in  the  transport  of  the  electricity.  The  uranium 
rays  fail  to  excite  fluorescence  (for  example,  in  barium  platino- 
cyanide),  but  this  almost  certainly  is  because  of  their  excessively 
Jeeble  character  compared  with  the  X-rays  from  an  ordinary  tube. 
Eutherford  has  also  conclusively  shown  (Phil.  Mag.,  1899, 
V.,  47,  p.  Ill,  that  the  uranium  rays  are  like  X-rays,  non- 
polarisable  and  non -refrangible. 


FIG.  2. — Two  Gold-leaf  Electroscopes,  Uncharged  and  Charged. 

It  will  be  convenient  at  the  present  stage  to  anticipate  the 
results  dealt  with  in  Chapter  V.  to  the  extent  of  giving 
a  general  account  of  the  nature  of  the  rays  from  uranium 
and  the  other  radio-active  substances.  These  radiations  have 
been  analysed  into  three  distinct  types,  which  are  differentiated 
in  the  first  instance  by  their  power  of  penetrating  matter. 
The  three  types  have  been  termed  respectively  a,  /3  and 
7- rays.  The  former  are  so  feebly  penetrating  that  they 
are  completely  stopped  by  a  single  sheet  of  note  paper  or 
by  a  few  centimetres  of  air.  The  /?-rays  resemble  ordinary 
X-rays  in  penetrating  power,  and  pass  with  ease  through  thin 
metal  foil,  glass,  &c.,  but  would  be  nearly  all  stopped  by 


RADIATION  PHENOMENA.  1$ 

a  single  coin.  The  y-rays  are  by  far  the  most  penetrating 
kind  of  ray  known,  and  pass  through  a  pile  of  12  coins  with- 
out being  completely  absorbed.  In  each  case  it  has  been 
found  that  the  simple  law  is  approximately  followed,  the 
absorption  being  mainly  proportional  to  the  density  and 
independent  of  the  nature  of  the  matter.  Of  the  three  types 
the  a-rays  are  the  most  and  the  y-rays  the  least  important. 
The  y-rays  from  a  kilogramme  of  uranium,  for  example,  can  only 
just  be  experimentally  detected  by  the  most  sensitive  test 
known.  We  shall  see  that  Becquerel  has  proved  the  /2-rays 
to  be  identical  in  type  with  the  cathode  rays  of  the  Crookes 
tube.  The  difference  is  that  the  /3-rays  are  travelling  with  a 
far  higher  speed  than  the  cathode  rays  ever  attain,  and  in 
consequence  have  a  much  greater  power  of  penetrating  matter. 
The  proof  rests  upon  the  fact  that  the  £-rays  are  easily 
deviated  by  a  magnet  in  the  same  direction  as  the  cathode 
rays.  The  y-ray  is  not  at  all  deviated  by  a  magnet,  and  it  is 
probable  that  the  y-ray  stands  in  a  similar  relation  to  the 
/2-ray  as  the  X-ray  stands  to  the  cathode  ray,  being  produced 
when  the  charged  particle  which  constitutes  the  latter 
is  suddenly  accelerated  at  the  moment  of  its  expulsion. 
The  /3-rays  are  most  active  in  impressing  the  photographic 
plate.  They  represent,  however,  but  a  very  small  fraction  of 
the  total  radiation  if  the  ionisation  of  the  air  is  used  as 
the  basis  of  detection.  The  a-rays  are  mainly  operative  in 
producing  this  effect.  These  have  been  shown  by  Rutherford 
to  be  very  slightly  deviated  in  a  powerful  magnetic  field 
in  the  opposite  direction  to  the  cathode  or  /?-ray.  We  shall 
see  that,  like  the  latter,  they  are  caused  by  the  radial 
expulsion  of  particles,  which,  however,  carry  a  positive  instead 
of  a  negative  charge.  Moreover,  the  radiant  particle  of  the 
a-ray  is  by  no  means  small,  but  is  a  body  of  about  the  mass  of 
a  hydrogen  atom,  and  it  is  expelled  from  the  radio-active 
substance  with  the  colossal  speed  of  20,000  miles  a  second. 
In  the  a  and  /3  types  of  rays  we  have  exactly  the  realisation  of 
Newton's  corpuscular  theory  of  light.  At  first  they  were 
mistaken  for  a  type  of  extreme  ultra-violet  light  radiation,  and 
their  wave-lengths  were  actually  estimated.  These  experi- 
ments are  now  known  to  be  erroneous.  But  it  is  curious  that 
the  first  wave-form  of  radiation  known  was  initially  considered 


14  RADIO-ACTIVITY. 

to  be  due  to  the  emission  of  corpuscles,  and  that  the  first  type 
of  the  latter  class  of  radiation  was  conversely  mistaken  for  a 
peculiar  kind  of  wave  vibration. 

In  some  respects  these  recent  discoveries  may  be  said  to 
have  provided  an  additional  indication  of  the  scientific  intuition 
and  foresight  of  Newton.  To  the  triumphant  supporters  of 
the  electromagnetic  theory  of  light  the  corpuscular  theory 
must  have  appeared  too  crude  to  merit  consideration.  To-day 
we  know  that  its  conception  anticipated  by  three  centuries 
the  march  of  scientific  progress. 

This  chapter  was  designed  to  give  at  the  outset  a  review  of 
the  generally  accepted  theories  of  the  nature  of  the  many 
kinds  of  radiation,  -old  and  new,  which  come  within  the  range 
of  modern  physics.  Two  classes  of  radiation  have  been 
recognised,  and  yet  the  various  types  of  both  classes  seem  to 
be  distinguished  from  one  another  by  a  gradual  rather  than  a 
sudden  break  of  properties.  There  is  no  single  criterion 
which  serves  to  differentiate  an  ethereal  disturbance  from  a 
corpuscular  radiation,  except,  perhaps,  the  action  of  a  magnetic 
field.  Even  this  test  would  fail  in  the  easily-conceived  case 
of  an  uncharged  radiant  particle. 

The  power  of  ionising  a  gas,  which  is  a  common  characteristic 
of  the  newly  discovered  radiations,  has  recently  been  shown 
to  be  possessed  by  ultra-violet  light  of  extremely  short  wave- 
length (below  2,000  Angstrom  units).  The  explanation  of  the 
transparency  of '  insulators  to  electromagnetic  undulations  of 
the  character  of  light  would  seem  to  necessitate  that  the  X-rays 
and  the  y-rays  should  be  more  easily  absorbed  by  conductors 
than  by  insulators.  Yet  this  has  not  been  observed.  Indeed, 
from  their  properties,  these  two  types  seem  to  be  more  nearly 
allied  to  the  cathode  rays  and  the  a  and  /?-rays  from  the 
radio-active  substances  than  to  light.  These  considerations 
may  serve  to  show  that  the  present  division  adopted  is  some- 
what unsatisfactory,  and  that  probably  in  the  future  it  will  be 
seen  that  a  closer  connection  exists  between  the  corpuscular 
and  undulatory  forms  of  radiation  than  our  present  views  of 
the  relations  between  electricity  and  matter  reveal. 


CHAPTER  II. 


THE    RADIO-ACTIVE    ELEMENTS— URANIUM, 
THORIUM,  RADIUM,  POLONIUM  AND  ACTINIUM. 

Uranium  and  Thorium,  the  only  examples  of  Radio-activity  among  the 
known  Elements. — Abnormal  Radio-activity  of  Pitchblende. — Discovery 
of  Radium  and  Polonium. — Radio-activity  an  Atomic  Property  of 
Matter. — Radium. — Source. — Method  of  Extraction. — Atomic  Weight. 
— Spectrum. — Radio-active  Properties  contrasted  with  ordinary 
Material  Properties. — Polonium. — Work  of  Marckwald. — Actinium. — 
Work  of  Giesel. — The  Five  Radio-elements  and  their  Distinguishing 
Characteristics. — Source  of  the  Energy  of  Radio-activity. — The  Two 
Alternatives. 

Soon  after  M.  Becquerel's  discovery  of  the  radio-activity  of 
the  element  uranium,  Mine.  Curie  (Thesis  presented  to  the 
Faculte  des  Sciences  de  Paris,  VHem.  News,  1903,  p.  85,  et  seq.) 
made  a  careful  examination  of  practically  all  the  known 
elements,  and  found  that  one  only — viz.,  thorium — possessed 
the  new  property.  Schmidt  simultaneously  discovered  the 
radio-activity  of  thorium.  With  regard  to  the  other  elements, 
Mme.  Curie  states  that  if  they  possess  radio-activity  it  can 
only  be  to  an  extent  at  least  100  times  more  feeble  than  that 
possessed  by  uranium  and  thorium. 

The  compounds  of  these  two  elements,  when  examined  for 
their  power  of  ionising  a  gas  and  discharging  an  electroscope, 
are  found  to  possess  a  very  similar  degree  of  radio-activity. 
To  the  photographic  plate,  however,  thorium  is  several  times 
less  active  than  uranium.  This  result  we  now  know  is  to  be 
explained  by  the  fact  that  the  a-radiation  from  the  two  bodies 
is  of  similar  intensity,  but  the  /3-radiation  of  thorium  com- 
pounds is  much  feebler  than  in  the  case  of  the  uranium 
compounds. 


16  RA  DIO-A  C  Tl  VII Y. 

Although  none  of  the  known  elements  in  the  pure  state 
possessed  appreciable  radio-activity,  Mme.  Curie  observed  that 
certain  ores  of  uranium  and  thorium,  notably  the  pitchblendes, 
were  several  times  as  radio-active  as  the  element  uranium  or 
any  of  its  compounds.  The  following  is  a  list  of  the  minerals 
examined  and  their  activities  : — 


x  10-11  amperes. 
Pitchblende  (Johanngeorgen- 

stadt)   8-3 

Do.  (Joachimsthal) . .  7'0 

Do.  (Pzibran) 6'5 

Do.  (Cornwall)    ' 1-6 

Cleveite 1-4 

Chalcolite  ...  .   5-2 


xlO-11  amperes. 

Thorite    1-4 

Crangeite    2'0 

Monazite     0-5 

.Eschymte 0'7 

Fergusonite    0-4 

Samarskite     1-1 

Niobite    0-3 


Autunite 2-7        Carnotite    6-2 

The  method  of  measurement  will  be  considered  later,  but 
the  activity  is  expressed  by  the  current  in  amperes  which  was 
enabled  to  pass  through  the  air,  under  the  action  of  the  rays, 
from  a  layer  of  the  powdered  substance  of  area  64  sq.  cm. 

The  following  table  gives  the  radio-activity,  on  the  same 
scale,  of  uranium  and  its  pure  compounds  : — 

x  10-11  amperes. 

Metallic  uranium  (containing  carbon)    2-3 

Black  oxide,  U205 2-6 

Green  oxide,  U3O8 1-8 

Uranic  hydrate,  U03  .  2H20    0-6 

Sodium  uranate,  Na2U207    1'2 

Potassium  uranate,  K2U207 1-2 

Ammonium  uranate,  (NH4)2U207    1-3 

Uranium  sulphate,  U02SO4,  3H2O 0-7 

Potassium  uranyl  sulphate,  U02S04,  K0SO4,  2H2O 0'7 

Uranium  nitrate,  UO2(N03)2  .  6H20  . . " 0-7 

Phosphate  of  copper  and  uranium !  . .     0-9 

Uranyl  sulphide,  U02S     1-2 

It  will  be  seen  from  the  above  table  that  the  radio-activity 
depends  mainly  on  the  percentage  of  uranium  present  in  the 
compound,  although  the  density  and  state  of  division  of  the 
compound,  by  altering  the  proportion  of  the  rays  absorbed 
in  the  substance  itself,  exerts  an  influence  on  the  observed 
activity.  But  the  Johanngeorgenstadt  pitchblende,  which  pro- 
bably contains  about  70  per  cent,  of  uranium,  is  nearly  four- 
times  as  active  as  the  most  active  uranium  compound.  Mme. 
Curie  next  tried  whether  an  artificial  mineral,  prepared 
from  pure  materials,  would  show  any  higher  activity  than  the 


THE  RADIO-ACTIVE  ELEMENTS.  17 

materials  of  which  it  is  compounded.  She  chose  for  the 
purpose  chalcolite,  which  is  a  phosphate  of  copper  and  uranium, 
and  she  found  the  artificial  preparation  possessed  an  activity 
corresponding  to  its  composition — viz.,  0*9,  compared  with 
5*2,  the  activity  of  the  natural  mineral. 

Mme.  Curie  came  to  the  conclusion,  as  the  result  of  these 
experiments,  that  radio-activity  is  an  atomic  property  which  is 
independent  of  the  physical  or  chemical  state  of  the  active 
element,  but  is  proportional  only  to  the  quantity  of  active 
element  present,  when  compounds  of  similar  density  and 
states  of  division  are  compared.  On  this  view,  the  high 
activity  of  the  natural  minerals  examined  could  only  be 
explained  by  supposing  them  to  contain  a  new  element 
(since  none  of  the  known  elements  except  uranium  and 
thorium  are  radio-active)  many  times  more  active  than 
uranium,  which  had  hitherto  escaped  detection  by  the 
analyst. 

M.  and  Mme.  Curie  proceeded  to  the  decomposition  of 
pitchblende,  and  its  separation  by  chemical  analysis  into 
its  constituent  elements.  Each  preparation  was  tested  for 
radio-activity.  If  the  high  activity  of  pitchblende  is  due  to 
the  presence  of  highly  active  new  elements,  it  is  to  be  expected 
that  the  radio-activity  in  certain  of  the  fractions  will  increase 
at  the  expense  of  the  activity  of  the  others. 

The  expected  result  was  obtained,  and  it  was  found  that 
the  bismuth  and  the  barium  separated  from  pitchblende  were 
both  strongly  radio-active.  These  active  preparations  were 
first  termed  "radio-active  bismuth"  and  "  radio-active  barium" 
respectively,  but  afterwards,  when  it  became  clear  that  the 
activity  could  only  be  caused  by  the  presence  of  new  elements, 
the  names  "polonium"  and  "radium"  respectively  were 
given  to  the  latter. 

Radium. — It  will  be  convenient  to  consider  the  case  of 
radium  first,  in  the  discovery  of  which  M.  Bemont  was 
associated  with  M.  and  Mme.  Curie.  Radium  is  the  only 
new  radio-element  that  has  so  far  been  isolated  in  the  form 
of  pure  compounds,  or  which  has  been  found  to  give  a  new 
spectrum.  It  was  discovered  that  the  barium  separated 
from  pitchblende  possessed  marked  activity,  and,  if  the 
active  barium  chloride  was  fractionally  crystallised,  the 


18  RADIO-ACTIVITY. 

activity  tended  to  concentrate  in  the  least  soluble  fraction. 
The  active  barium  preparations  were,  as  soon  as  possible,  sub- 
mitted to  M.  Demarcay  for  spectroscopic  examination,  in  order 
that  the  hypothesis  of  the  existence  of  a  new  element  might 
be  confirmed.  In  the  first  specimens  a  new  line  in  the  ultra- 
violet (A  =  381'47/>t/>i)  of  considerable  intensity  was  detected, 
and,  as  preparations  were  obtained  of  higher  and  higher  activity, 
this  line  became  stronger  and  new  lines  appeared.  As  the 
fractionation  of  large  quantities  of  material  was  proceeded  with 
small  quantities  of  intensly  active  products  were  prepared,  in 
which  the  new  spectrum  predominated  and  only  the  three 
strongest  barium  lines  were  visible.  Finally  a  product  was 
obtained,  in  the  spectrum  of  which  the  presence  of  barium  lines 
was  scarcely  detectable. 

The  quantities  of  pure  radium  compounds  so  obtained  are 
excessively  small.  A  few  tenths  of  a  gramme  only  of  radium 
chloride  can  be  extracted  from  a  ton  of  pitchblende,  and  this 
is  in  the  ratio  of  one  part  to  several  millions  of  the  original 
mineral.  On  the  other  hand,  the  radio-activity  of  the  pure 
compound  is  correspondingly  increased,  and  the  tiny  quantity 
extracted  from  a  ton  of  ore  retains  in  concentrated  form  the 
greater  part  of  the  radio-activity  of  the  original  mineral. 
Weight  for  weight,  the  radium  compounds  are  at  least  a 
million  times  more  active  than  the  compounds  of  uranium 
and  thorium.  This  numerical  comparison  hardly  conveys  a 
true  impression  of  the  relative  activities.  A  clearer  idea  is 
given  by  a  concrete  example.  If  a  radium  compound 
produced  a  given  effect,  for  example,  on  a  photographic 
plate,  in  one  second,  a  similar  weight  of  uranium  com- 
pound would  take  several  weeks  to  produce  the  same  effect. 
This  is  illustrated  by  Figs.  3  and  4.  The  first  illustrates 
a  radiograph  of  an  aluminium  medal  taken  with  uranium 
rays  in  which  probably  several  grammes  of  a  uranium 
compound  were  employed,  and  the  exposure  given  was 
14  days.  The  second  illustrates  a  negative  obtained  by 
merely  writing  slowly  on  a  photographic  plate  with  a 
tube  containing  a  few  thousandths  of  a  gramme  of  a  pure 
radium  preparation. 

The  use  made  by  M.  and  Mme.  Curie  of  the  property  of 
radio-activity  as  a  means  of  detecting  and  separating  new 


FIG.  3.— Radiograph  of  an  Aluminium  Medal  taken  by  M.  Becquerel 

with  the  Uranium  Rays.     Exposure :  A  fortnight. 

(From  "Rapports  du  Congres  International  de  Physique,  1900," 

Tome  III.,  p.  51.) 


FIG.  4. — Negative  produced  by  Writing  with  a  Glass  Tube  containing  a 
few  milligrammes  of  Pure  Radium  Bromide  on  a  Photographic  Plate 
wrapped  in  Black  Paper. 

"  Radio- Activity.-  [To  face  pagelS. 


THE  RADIO-ACTIVE  ELEMENTS.  19 

elements  is  strictly  analogous  to  the  use  of  the  spectroscope 
made  by  Bunsen  and  Kirchhoff  for  the  same  purpose,  in  their 
discovery  forty  years  ago  of  two  new  elements,  rubidium  and 
caesium.  Thus  44,200kg.  of  Diirkheim  water  were  worked  up 
by  these  investigators  and  7*272  grammes  of  caesium  chloride 
obtained  therefrom,  the  sole  guide  to  the  existence  and,  in 
the  early  stages  of  the  separation,  to  the  analytical  behavour 
of  this  new  element  being  the  two  bright  blue  lines  in  the 
flame  spectrum  of  caesium. 

Mme.  Curie  states  that  the  strongest  line  of  radium  cannot  be 
observed  with  specimens  less  radio-active  than  fifty  times  that 
of  uranium.  Since  radio-activity  1501000th  of  this  can  be  directly 
detected  (compare  Strutt,  Phil.  Mag.,  1903,  VI.,  5,  p.  683), 
it  follows  that  for  radium  the  radio-active  test  is  150,000 
times  more  sensitive  than  the  spectroscopic.  Nevertheless, 
Demar9ay  classes  radium  among  the  elements  which  possess 
the  most  sensitive  spectrum  reaction.  In  pitchblende  itself 
the  quantity  of  radium  is  probably  about  one-twentieth  of  the 
minimum  quantity  detectable  by  the  spectroscope.  Recently 
it  has  been  demonstrated  that  radium  is  very  widely  distri- 
buted in  nature.  The  test  used  is  not  the  direct  radiation, 
but  depends  on  the  fact  that  radium  produces  an  emanation, 
or  radio-active  gas,  whose  presence  can  be  detected  in  almost 
inconceivably  minute  quantities.  The  presence  of  the  radium 
emanation,  and,  by  deduction,  the  existence  of  radium,  has  been 
detected  in  the  soil  of  many  localities,  in  tap  water  and  in 
various  springs,  also  (compare  J.  J.  Thomson,  Cambridge 
Phil.  Soc.  Proceedings,  February  15th,  1904)  in  the  com- 
monest materials,  such  as  flour,  the  sand  of  the  sea  shore,  &c. 
In  fact,  it  is  hardly  too  much  to  expect  that,  as  the  methods  of 
investigation  are  improved,  and  sufficiently  large  quantities  of 
material  are  worked  with,  radium  will  appear  as  a  universal 
constituent.  This  is  mainly  due  to  the  fact  that  the  means  of 
detecting  it  are  almost  infinitely  more  sensitive  than  those 
possible  for  any  of  the  older  known  elements.  Conversely, 
the  property  of  radio-activity  has  supplied  the  chemist  with  so 
powerful  a  new  weapon  in  the  search  for  excessively  rare 
elements  possessing  the  property  that  a  great  extension  of 
our  knowledge  in  this  direction  is  to  be  anticipated  in  the 
near  future. 

c2 


20  EADIO- ACTIVITY. 

The  principle  underlying  this  method  of  research,  and  which 
is  also  at  the  root  of  the  important  theoretical  advances  that 
have  been  so  quickly  made  in  the  subject,  depends  upon  the 
fact  that  radio-activity  is  an  atomic  property.  When  the  prin- 
ciple was  first  enunciated  by  Mme.  Curie,  it  connoted  that  the 
activity  of  any  radio-element  was  not  influenced  by  its  state 
of  chemical  combination  or  by  the  conditions  under  which  it 
was  tested.  Thus  any  mineral  containing  uranium  or  thorium 
is  radio-active  without  reference  to  the  other  inactive  elements 
it  contains,  or  to  the  conditions  of  its  past  history.  We  now 
know  that,  if  proper  precautions  are  taken,  the  radio-activity 
of  a  substance  furnishes  a  good  quantitative  measure  of  the 
amount  of  the  radio-element  it  contains.  The  radio-activity 
of  matter  is  a  property  which  can  neither  be  increased  nor 
diminished  by  any  artificial  means. 

Matter,  strictly  speaking,  possesses  only  one  other  "atomic 
property,"  and  that  is  mass,  which,  like  radio-activity,  is  an 
additive  property  of  the  atoms,  each  constituent  atom  contri- 
buting its  quota  without  possibility  of  its  being  changed  by  any 
agency  we  know  of.  This  is  shown  by  the  fact  that  mass,  or,, 
as  it  is  usually  measured,  weight,  is  the  only  property  that 
can  be  used  universally  as  the  basis  of  chemical  analysis. 
Of  other  properties  spectrum  reactions  represent  probably 
the  nearest  approach  to  an  atomic  property,  for,  within 
certain  limits  of  temperature,  the  spectrum  of  an  element 
is  the  same  whatever  the  compound  employed.  But  the 
spectrum  reaction  is  not  an  additive  property,  and  the 
intensity  of  a  spectrum  is  by  no  means  a  sure  indication  of 
the  quantity  of  element  present,  for  many  spectra  are- 
"  masked "  by  comparatively  small  quantities  of  impurities. 
Radio-activity  is,  of  course,  distinguished  from  the  property  of 
mass  or  weight  in  being  a  very  special  property  of  a  certain 
few  types  of  atoms,  and  not  a  universal  property  of  all. 

In  spite  of  the  discovery  of  many  new  facts,  which  at 
first  sight  seemed  to  contradict  the  view  that  radio-activity 
is  an  atomic  property,  there  is  no  doubt  of  its  truth 
at  the  present  time.  It  has  not  been  found  possible  to 
influence  radio-activity  by  any  known  agency,  any  more 
than  it  has  been  found  possible  to  alter  weight,  and  the 
more  free  from  disturbing  causes  (such  as  absorption). 


THE  RADIO-ACTIVE  ELEMENTS.  21 

the  measurements  are,  the  more  accurately  this  result 
holds  good.  Hence  radio-activity  is  fitted  in  every  respect  as 
a  property  upon  which  a  method  of  quantitative  chemical 
analysis  could  be  based.  We  shall  see  later  that  radio-activity 
furnishes  a  means  of  qualitative  analysis  also.  There  are 
certain  peculiarities  which  enable  us  instantly  to  recognise 
thorium,  or  radium,  from  uranium  (even  were  all  these  elements 
mixed)  by  the  character  of  their  radio-activity,  without  perfor- 
ming a  single  chemical  or  spectroscopic  test  with  the  substance. 
Conversely,  by  an  examination  of  a  radio-active  mineral,  one 
can  be  sure,  before  a  single  chemical  test  is  employed,  whether 
it  contains  any  new  and  hitherto  unrecognised  active  elements, 
or  whether  its  activity  is  due  to  the  known  radio-elements.  The 
finest  and  best  methods  of  chemical  analysis  do  not  surpass  in 
ease  of  operation  and  certainty  the  methods  of  "  radio-active 
analysis,"  whereas  the  latter  tests  are  applicable  for  quantities 
millions  of  times  less  than  the  former. 

Source  of  Radium. — So  far  radium  has  only  been  found  in 
notable  amount  in  minerals  containing  uranium,  and  very  few 
of  these  appear  to  contain  a  sufficient  quantity  to  repay  ex- 
traction. Not  all  pitchblendes  are  equally  rich  in  the  element ; 
for  example,  the  Cornish  pitchblende,  although  it  contains  a 
high  percentage  of  uranium,  possesses  such  a  low  activity  (1-6 
compared  to  8-3  for  the  Johanngeorgenstadt  ore)  that  it  can 
only  contain  little,  if  any,  of  the  element.  Practically  the 
ore  from  Joachimsthal  provides  the  whole  of  the  available 
supply.  A  method  for  the  accurate  estimation  of  radium 
in  ores  will  be  given  later,  after  its  properties  have  been 
more  closely  studied.  The  quantity  obtainable  even  from  the 
best  ores  is  excessively  small.  Mme.  Curie  states  that  from 
2  tons  of  residues  from  the  Joachimsthal  ore  she  obtained 
a  few  hundredths  of  a  gramme  of  the  pure  chloride  and  a 
few  tenths  of  a  gramme  of  considerably  less  active  material. 
Oiesel,  who  has  probably  prepared  the  largest  quantities  of 
concentrated  radium  compounds,  obtains  about  O25  gramme 
of  pure  radium  bromide  from  1  ton  of  the  residues  of  the 
pitchblende  after  the  commercially  valuable  uranium  has  been 
extracted. 

Method  of  Extraction. — The  following  method  of  treatment 
of  the  residues  from  the  Joachimsthal  ores  has  been  worked 


22  RADIO-ACTIVITY. 

out  by  Debierne  and  is  taken  from  Mme.  Curie's  Thesis  : — To 
extract  the  uranium,  the  ore  is  roasted  with  sodium  carbonate, 
lixiviated  with  warm  water  and  then  with  dilute  sulphuric 
acid,  when  the  uranium  passes  into  solution.  The  insoluble 
part  consists  of  the  sulphates  of  lead  and  calcium,  alumina, 
silica  and  iron  oxide,  together  with  greater  or  less  quantities 
of  nearly  all  the  metals.  These  residues  possess  an  activity 
four  and  a-half  times  that  of  uranium,  and  constitute  the  raw 
material  used  for  the  extraction  of  radium.  The  insoluble 
sulphates  are  converted  into  carbonates  by  boiling  with  a  con- 
centrated solution  of  carbonate  of  soda,  and  the  soluble  sodium 
sulphate  produced  is  removed  by  repeated  washing.  The 
residue  is  treated  with  hydrochloric  acid,  which  dissolves  most 
of  it,  including  the  polonium  and  actinium,  but  the  radium 
remains  undissolved,  as  unconverted  sulphate.  It  is  washed 
with  water,  again  boiled  with  concentrated  sodium  carbonate 
(which  completes  the  transformation  of  the  sulphates  into  car- 
bonates), again  thoroughly  washed,  and  then  treated  with  dilute 
hydrochloric  acid  free  from  sulphuric  acid.  Polonium  and 
actinium  are  still  present  in  the  solution,  from  which  the 
radium  and  barium  are  removed  by  precipitating  with  sulphuric 
acid.  From  1  ton  of  residues  10kg.  to  20kg.  of  crude  sulphates 
are  thus  obtained  of  activity  about  60  times  that  of  uranium, 
and  these  contain  calcium,  lead,  iron  and  a  trace  of  actinium. 
The  sulphates  are  transformed  into  chlorides  as  before,  and  the 
solution  treated  with  sulphuretted  hydrogen,  filtered,  oxidised 
with  chlorine  and  precipitated  with  ammonia.  The  activity  of 
the  precipitated  hydrates  and  oxides  is  due  to  actinium.  The 
filtrate  is  precipitated  with  sodium  carbonate,  and  the  precipitate 
washed  and  converted  into  chlorides,  evaporated  to  dryness,  and 
the  chlorides  washed  with  concentrated  hydrochloric  acid,  which 
removes  calcium.  The  purified  chlorides  of  barium  and  radium 
thus  obtained  possess  an  activity  of  about  60.  Eight  kilos  are 
obtained  from  1  ton  of  residues.  At  this  stage  the  material 
leaves  the  factory,  and  is  now  fractionated  in  the  laboratory. 

Mme.  Curie's  system  was  based  upon  the  fact  that  if  the 
mixed  chlorides  are  dissolved  in  water,  raised  to  the  boiling 
point,  and  allowed  to  cool,  the  small  part  that  crystallises  out 
is  five  times  more  active  than  the  part  left  in  solution.  By 
successive  repetitions  of  this  process  the  mixture  is  divided 


THE  RADIO-ACTIVE  ELEMENTS.  23 

into  two  parts— the  one  consisting  of  the  great  bulk  of  the 
material,  which  may  be  obtained  only  one-tenth  as  active  as 
uranium,  the  other  consisting  of  continuously  diminishing 
quantities  of  preparations  with  correspondingly  increasing 
activity.  As  the  fractionation  advances  it  is  convenient  to  dis- 
solve in  water  containing  gradually  increasing  quantities  of 
hydrochloric  acid.  The  salts  are  less  soluble  in  the  latter  than 
in  water,  and  larger  volumes  of  solution  are  obtained.  When 
the  proportion  of  radium  increases  to  a  certain  point,  the 
crystals  of  mixed  barium  and  radium  chloride  become  coloured 
after  a  few  hours,  the  colour  disappearing  when  the  crystals 
are  re-dissolved.  A  maximum  coloration  is  reached  when  the 
radium  present  is  of  a  certain  concentration,  and  then  again 
diminishes  as  the  preparation  becomes  more  concentrated. 
This  serves  as  a  check  on  the  progress  of  the  fractionation. 

Giesel  (Berichte  der  Deutschen  Chemischen  Gesellscliaft,  1902, 
p.  3,609)  has  improved  upon  the  method  by  using  the  bromide 
instead  of  the  chloride  for  fractionation.  He  states  that  eight 
crystallisations  suffice  to  remove  the  easier  soluble  barium 
bromide,  leaving  the  radium  bromide  approximately  pure.  But 
it  is  probable  that  to  approach  the  purity  ultimately  attained 
by  Mme.  Curie  with  her  method  very  many  more  fractionations 
would  be  required.  Such  a  degree  of  purity  is,  however,  only 
required  in  certain  special  cases,  and  Giesel's  method  furnishes 
preparations  with  sensibly  the  maximum  obtainable  radio- 
activity. Giesel  discovered  that  salts  of  radium  impart  to 
the  colourless  flame  of  a  bun  sen  burner  a  fine  carmine  red 
colour,  which  serves  as  a  useful  control  over  the  progress 
of  fractionation.  In  this  respect  radium  shows  its  relation- 
ship to  the  other  alkaline-earths,  which  all  yield  fine  flame 
colorations. 

Atomic  Weight. — The  atomic  weight  of  radium  has  been 
determined  by  Mme.  Curie  with  the  preparations  obtained 
during  the  progress  of  the  fractionations.  The  method  em- 
ployed was  to  precipitate  the  chlorine  in  a  known  weight  of 
the  chloride  with  silver  nitrate,  and  to  weigh  the  silver 
chloride  obtained.  Besides  being  accurate  and  applicable  to 
very  small  quantities  of  material,  the  method  possesses  the 
advantage  of  retaining  the  radium  in  the  soluble  form,  as 
nitrate,  after  the  performance  of  the  operation. 


-24 


RADIO- ACTIVITY. 


The  atomic  weight  of  preparations  of  activity  230  and  600 
times  as  active  as  uranium  was  indistinguishable  from  that 
of  barium — viz.,  137.  With  a  preparation  of  activity  3,500 
the  number  140  was  obtained,  showing  a  small  but  distinct 
difference.  Another  sample  of  activity  7,500  gave  the  num- 
ber 145-8,  while  two  samples  of  very  high  activity,  each  of 
the  order  of  one  million  times  that  of  uranium,  gave  173*8  and 
225  respectively.  In  the  latter  the  barium  could  only  just  be 
detected  spectroscopically.  A  mean  of  three  determinations 
of  the  purest  sample  prepared  also  gave  the  number  225. 

It  may  be  noted  that  the  radio-activity  of  pure  radium  salts 
is  generally  taken  to  be  of  the  order  of  a  million  times  that  of 
uranium,  but  the  exact  value  is  unknown.  This  is  because  it 
is  experimentally  impossible  to  directly  compare  together 
activities  which  are  so  widely  different.  An  absolute  measure 
in  electrical  units  of  the  radio-activity  of  pure  radium  salts  has 
not  yet  been  published. 

The  value  225  for  its  atomic  weight  places  radium  in  the 
position  of  the  third  heaviest  element  known,  the  two  heavier 
being  the  other  radio-elements — thorium  232  and  uranium  238. 
It  is  surely  not  merely  a  coincidence  that  the  three  elements 
distinguished  by  their  radio-activity  should  possess  the  three 
heaviest  atomic  weights.  Except  in  this  respect  they  are 
chemically  very  dissimilar,  radium  being  divalent  like  barium, 
thorium  tetravalent  like  tin,  and  uranium  hexavalent  like 
tungsten.  On  the  other  hand,  these  elements,  which  they  most 
resemble,  show  no  trace  of  radio-activity. 

Spectrum. — The  following  table  shows  the  measurements  of 
DemarQay  of  the  spark  spectrum  of  radium  between  the  limits 
A  — 500'0  and  A^SSO'O//./*.  The  intensity  of  the  strongest 
line  is  put  at  16  : — 


Blue 


In  addition  there  are  two  well-marked  misty  bands  in  the 
spectrum,  and  in  this  and  other  characteristics  the  spectrum  is 
completely  similar  to  those  of  the  other  alkaline  earth  metals. 


X 

Intensity. 

X 

Intensity. 

482-63 

10 

(  460-03 

3 

472-69 

5 

Violet  \  453-35 

9 

469-98 

3 

1443-61 

8 

469-21 
468-30 

7 
14 

TT1,       (434-06 
Ultra-  !  381.47 

12 

16 

464-19 

4 

violet  (3g4.gg 

12 

THE  RADIO-ACTIVE  ELEMENTS. 


Eunge  and  Precht  (Phil.  Mag.,  1903,  VI.,  5,  p.  476)  have 
examined  the  spark  spectrum  of  radium  in  a  magnetic  field, 
and  have  found  that  the  strongest  lines  are  exactly  analogous 
in  their  behaviour  to  the  strongest  lines  of  barium,  and  to  the 
corresponding  lines  of  the  related  elements  magnesium,  calcium 
and  strontium.  Like  the  latter,  the  radium  lines  may  be 
grouped  into  three  series,  a  pair  of  lines  in  each  series.  In 
the  second  series  of  each  element  there  occurs,  besides  the 
two  lines,  a  third,  which  they  term  a  "  satellite."  In  calcu- 
lating the  distance  apart  of  the  lines  of  each  series,  the 
oscillation-frequency  which  is  the  reciprocal  of  the  wave-length 
is  employed.  Runge  and  Precht  find  that  the  distance  apart 
between  the  two  lines  of  the  first  and  third  series,  and  of  one 
of  the  lines  and  the  "  satellite  "  in  the  second  series,  is  the 
same  for  each  element  and  different  for  different  elements. 
The  following  table  shows  the  corresponding  lines  in  the  spectra 
of  magnesium,  calcium,  strontium,  barium  and  Tadium,  ex- 
pressed in  Angstrom  units  (/*=  10  Angstrom  units) : — 


"  — 

Mg. 

Ca. 

Sr. 

Ba. 

Ka. 

Primary  series  

/  2,803 

3,969 

4,216 

;    4,934 

4,682 

1  2,796 

3,934 

4,078 

4,554 

3,815 

(    - 

3,181 

3,475 

4,166 

4,436 

1st  secondary  series   .... 

1  2,798 

3,179 

3,465 

!    4,131 

4,341 

(2,791 

3,159 

3,381 

|    3,892 

3,650 

2nd  secondary  series  .... 

/2,937 
\  2,929 

3,737 
3,706 

4,306 
4,162 

4,900 
!    4,525 

5,814 
4,533 

The  following  is  a  table  of  the  distances  apart  of  the  three 
pairs  in  the  case  of  radium  measured  on  the  scale  of  frequency  : — 


— 

X 

108/X 

Difference. 

Primary  series  

/  4,682-35 

21,356-8  \ 

4  858*3 

1st  secondary  series 

\    3,814-59 
/  4,436-45 

26,215-1  / 
22,5405    I 

4  858  5 

2nd  secondary  series 

V   3,649-77 
/  5,813-9 

27,399-0  / 
17.200-2  \ 

4  858-6 

\   4,533-33 

22,058-8  / 

Now,  in  a  series  of  elements  of  the  same  family  in  the  periodic 
table,  the  distances  apart  of  the  lines  increases  with  the 
atomic  weight  of  the  element,  and  from  the  values  in  the 
case  of  Mg,  Ca,  Sr  and  Ba  Runge  and  Precht  have  extrapo- 
lated for  the  case  of  radium,  and  obtained  for  the  atomic 


26  RADIO-ACTIVITY. 

weight  of  radium,  the  number  257' 8.  This  is  considerably 
above  the  experimental  number  found  by  Mme.  Curie,  and  the 
discrepancy  has  not  been  explained.  This  matter  needs  further 
investigation,  but  for  the  present  we  must  accept  the  experi- 
mental value,  225,  especially  as  it  is  that  required  by  the 
periodic  law. 

The  position  is  a  very  interesting  one.  Both  the  chemical 
and  spectroscopic  evidence  point  most  strongly  to  the  con- 
clusion that  radium  is  a  heavy  representative  of  the  alkaline- 
earth  family  of  element.  If  old  tables  of  the  periodic  law  are 
consulted  there  are  two  vacancies  possible  beyond  barium,  the 
then  heaviest  known  member  of  the  family.  One  corresponds 
to  atomic  weight  about  172,  the  other  to  225.  The  former  is 
too  small  to  agree  with  either  determination,  and  need  not  be 
considered,  whereas  the  latter  is  identical  with  Mme.  Curie's 
number.  If  the  value  257 '8  indicated  by  the  spectroscopic 
evidence  were  correct,  radium  would  appear  in  the  periodic 
table  allied  to  the  mercury-cadmium  family  of  elements  and 
not  to  the  alkaline-earths. 

The  line  4,826*14  in  the  radium  spark  spectrum  is  stated  by 
Eunge  and  Precht  to  be  the  one  most  prominent  in  the  flame 
spectrum,  and  is  analogous  to  the  strongest  flame  lines 
Ba  5,535,  Sr  4,607,  Ca  4,426,  for  all  these  lines  are  resolved 
into  triplets  by  a  magnetic  field,  the  lines  in  each  case  being 
equidistant  from  one  another  on  the  scale  of  frequency.  We 
thus  see  that  the  spectrum  of  radium,  when  subjected  to  the 
strictest  investigation  by  the  modern  methods  of  spectroscopy, 
is  the  normal  spectrum  to  be  expected  for  an  alkaline-earth 
element  heavier  than  barium.  Eadium  shows  in  its  spectrum 
no  indication  of  any  special  peculiarities  which  might  be  con- 
nected with  its  many  unique  radio-active  properties,  and  this  is 
the  more  surprising  when  it  is  considered  that  the  methods  of 
spectroscopy  have  thrown  a  great  deal  of  light  on  the  internal 
mechanism  of  the  chemical  atom. 

In  marked  contrast  to  its  strictly  normal  character  as  a 
chemical  element  occupying  a  definite  and  predicted  place  in 
the  periodic  table,  radium  possesses  in  addition  a  totally 
different  set  of  properties,  which  make  it  in  many  ways  the 
most  extraordinary  substance  known.  These  will  be  generally 
referred  to  as  its  radio-active  properties,  in  contradistinction 


THE  RADIO-ACTIVE  ELEMENTS.  27 

to  the  other  set,  which  will  be  termed  the  ordinary  material 
properties.  There  is  thus  a  double  nature  to  the  radio-elements, 
according  as  their  ordinary  material  properties  or  their  radio- 
active properties  are  considered.  We  shall  see  later  that  it  is 
probable  that  the  same  set  of  atoms  is  not  concerned  in  the 
two  cases,  the  ordinary  material  properties  being  caused  by 
what  may  be  called  the  normal  atoms,  which  are  in  over- 
whelming numerical  majority,  while  the  radio-active  properties 
are  caused  by  a  very  minute  and  constant  fraction  of  the  total 
in  a  very  critical  and  unusual  state. 

It  will  be  convenient  to  pass  in  review  in  a  preliminary 
manner  some  of  the  most  important  of  the  radio-active 
properties  of  radium.  The  radiations,  even  from  a  few  milli- 
grammes, are  of  an  extraordinarily  powerful  character.  We  have 
seen  that  they  can  be  divided  into  three  classes,  in  the  order 
of  their  penetrating  power.  The  a  or  non-penetrating  type 
are  stopped  by  less  than  an  inch  of  air.  They  instantly 
discharge  an  electroscope  or  an  electrified  silk  tassel,  and  cause 
zinc  sulphide  to  phosphoresce  very  brilliantly.  The  scintilla- 
tions of  Sir  W.  Crookes'  "  spinthariscope"  are  brought  about  by 
the  impact  of  the  a-rays  on  a  zinc  sulphide  screen.  Owing  to 
their  enormous  kinetic  energy  and  the  ease  with  which  they 
are  stopped  by  material  obstacles,  the  a-rays  are  the  main  cause 
of  the  heat  evolution  from  radium.  By  far  the  greater  part  of 
the  a  radiations  are  absorbed  in  the  solid  compounds  of 
radium,  and  their  energy  is  converted  into  heat.  In  this  way 
radium  liberates  enough  heat  every  hour  to  raise  its  own 
weight  of  water  from  the  freezing  to  the  boiling  point.  They 
continuously  maintain  themselves  a  few  degrees  above  the 
temperature  of  their  surroundings.  The  a-rays  bring  about 
chemical  decompositions,  the  most  remarkable  of  which  is  the 
decomposition  of  water  into  its  constituent  elements.  A 
gramme  of  radium  in  aqueous  solution  liberates  10  cubic  cm. 
of  hydrogen  and  oxygen  per  day  continuously. 

The  /2-rays  of  radium  pass  with  ease  through  plates  of 
copper  and  other  metals  of  the  thickness  of  a  visiting  card,  and 
will  instantly  affect  a  photographic  plate  or  produce  brilliant 
fluorescence  on  a  crystal  of  barium  platino-cyanide  on  the 
other  side.  As  they  carry  away  a  negative  charge,  the 
radium,  under  favourable  circumstances,  is  continuously  kept 


28  RADIO-ACTIVITY. 

charged  with  positive  electricity.  The  y-rays  of  radium  will 
affect  a  photographic  plate  and  produce  visible  fluorescence 
on  a  screen  of  platino -cyanide  through  metal  screens  over  an 
inch  thick,  or  through  a  pile  of  12  coins.  In  addition,  there 
occurs  simultaneously  a  continuous  production  of  a  radio- 
active emanation,  or  gas,  the  absolute  amount  of  which  is  so  small 
that  it  can  scarcely  be  directly  perceived.  Yet  this  invisible 
quantity  of  gas  emits  enough  energy  to  give  astonishing  indi- 
cations of  its  existence.  The  quantity  of  emanation  obtainable 
from  a  few  milligrammes  of  radium  salt  will  keep  a  phosphores- 
cent mineral  like  willemite  continuously  luminous  for  several 
days  or  even  weeks,  to  a  sufficient  extent  to  be  visible  even  in  a 
brightly  lighted  room. 

A  fuller  consideration  of  this  property  and  of  others  con- 
nected with  it  must  be  reserved  till  later  (Chapter  VII., 
et  seq.).  At  this  stage  the  point  must  be  emphasised  that, 
in  spite  of  the  properties  above  mentioned,  the  radio-activity 
of  radium  appears  in  no  fundamental  point  different  from  that 
of  uranium  and  thorium.  It  is  mainly  a  matter  of  the 
phenomena  in  the  former  case  being  exhibited  to  an  extra- 
ordinarily greater  degree  than  in  the  latter  cases. 

Polonium. — Besides  radium  there  is  strong  evidence  that 
there  exists  in  pitchblende  two  other  new  radio-active  bodies, 
polonium  and  actinium,  and,  although  neither  of  these  has  yet 
been  obtained  in  sufficient  quantity  to  give  a  spectrum  reaction 
or  any  other  evidence  of  their  presence,  except  their  radio- 
activity, the  peculiar  nature  of  the  latter  in  each  case  leaves 
no  doubt  that  each  is  a  specific  new  form  of  matter,  whose 
ultimate  separation  may  be  confidently  expected.  In  the  frac- 
tionation  of  pitchblende  by  M.  and  Mme.  Curie,  the  polonium 
appeared  with  the  bismuth  in  the  second  group  of  metals 
precipitated  from  acid  solution  by  hydrogen  sulphide.  It 
resembles  bismuth  in  analytical  properties,  but  it  can  be 
concentrated  by  special  methods  even  from  the  bismuth,  which 
shows  that  it  is  chemically  distinct.  Mme.  Curie  describes 
three  methods  :  (1)  By  subliming  the  active  sulphide  in  vacuo, 
the  active  part,  being  the  more  volatile,  tends  to  deposit  on 
the  cooler  parts  of  the  tube.  (2)  By  fractional  precipitation 
of  the  solution  of  the  nitrates  with  water  it  is  found  that  the 
basic  nitrate  precipitated  is  more  active  than  the  fraction  which 


THE  RADIO-ACTIVE  ELEMENTS.  29 

remains  dissolved.  (3)  By  precipitation  of  the  chlorides  in 
strong  hydrochloric  acid  solution  with  hydrogen  sulphide  the 
fraction  precipitated  as  sulphide  is  again  the  more  active. 

So  far  Mme.  Curie  has  not  succeeded  in  obtaining  her 
preparations  free  from  bismuth.  Its  radio-activity  is  peculiar 
in  two  respects  :  (1)  It  comprises  only  a  or  non-penetrating 
rays,  the  penetrating  /3-rays  being  completely  absent.  In  this 
respect  it  is  sharply  distinguished  from  the  other  radio-active 
substances,  uranium,  thorium,  radium  and  actinium.  (2)  Its 
radio-activity  is  not  permanent,  but  decays  slowly  with  the 
lapse  of  time,  falling  to  half  its  original  value  at  the  expiration 
of  about  one  year.  The  discussion  of  this  important  point, 
which  throws  a  great  light  on  the  nature  of  polonium,  and  why 
it  has  so  far  been  impossible  to  separate  it  as  a  distinct  element, 
must  be  reserved  until  the  numerous  other  cases  of  temporary 
activity  which  are  now  known  have  been  considered. 

Recently  Marckwald  (Berichte  derDeutschen  Chemischen  Gesell- 
schaft,  1902,  p.  2,285)  has  shown  that  polonium  may  be  easily 
separated  from  the  bismuth  extracted  from  pitchblende  residues 
by  immersing  a  polished  plate  of  bismuth  in  the  solution.  The 
whole  of  the  active  substance  is  deposited  in  highly  concen- 
trated form  on  the  plate,  while  the  solution  is  left  inactive.  The 
deposit,  which  is  metallic,  can  be  easily  detached,  and  weighs  but 
a  few  milligrammes,  which,  however,  when  dissolved  still  shows 
the  presence  of  bismuth.  In  a  later  communication  (ibid., 
p.  4,239)  the  same  investigator  has  shown  that  the  metallic 
deposit  more  nearly  resembles  tellurium  than  bismuth  in  its 
chemical  nature,  and  he  has  proposed  the  name  radio-tellurium 
for  his  preparation,  believing  it  to  be  distinct  from  Mme. 
Curie's  polonium.  He  finds  that  it  is  precipitated  out  of  its 
solutions  by  stannous  chloride  as  a  black  precipitate,  and 
recommends  this  method  as  giving  far  purer  preparations  than 
that  first  employed.  The  activity  of  these  is  far  higher  than 
the  earlier  products,  but  the  weight  obtained  is  correspond- 
ingly smaller.  In  a  still  later  communication  (ibid.,  1903, 
p.  2,662)  Marckwald  gives  some  very  interesting  details  of  the 
actual  amount  of  the  new  substance  present  in  pitchblende. 
2,000kg.  of  the  latter  yielded  6kg.  of  bismuth  oxychloride, 
which  gave  1'5  grammes  of  the  tellurium-like  precipitate  with 
stannous  chloride.  He  now  finds  that  this  is  almost  entirely 


30  RADIO-ACTIVITY. 

ordinary  inactive  tellurium,  which  he  separates  from  the  active 
product  by  precipitating  the  chloride  with  hydrazin  hydro- 
chloride  in  not  too  acid  solution.  The  active  substance  remains 
in  the  filtrate,  and  after  precipitating  with  stannous  chloride 
was  found  to  weigh  4  milligrammes  !  Even  so,  the  author  does 
not  consider  it  proved  to  be  a  homogeneous  substance.  He 
states  that  from  T^^h  milligramme  of  this  preparation  he  has 
obtained  sufficient  light,  by  means  of  the  phosphorescence  pro- 
duced by  the  rays  in  a  preparation  of  zinc  sulphide,  to  be 
plainly  visible  to  an  audience  of  several  hundred  people. 

There  seems  to  be  no  justification  for  supposing  that  the 
active  constituent  of  Marckwald's  preparation  is  different  from 
that  of  Mme.  Curie.  Both  preparations  are  identical  in  their 
radio-active  properties  and  are  obtained  from  the  same  mineral. 
In  this  book  it  will  be  assumed  that  "radio-tellurium"  is 
identical  with  polonium.  For  a  full  discussion  of  this  point 
compare  Nature,  March  17,  1904,  p.  461. 

On  the  view  that  polonium  is  a  new  element,  this  work  of 
Marckwald  establishes  a  record  in  the  achievements  of  chemical 
analysis.  Before  it,  the  separation  of  xenon  from  the  atmo- 
sphere, one  part  of  the  former  in  between  10  and  100  millions 
of  the  latter,  by  Sir  William  Ramsay  and  Dr.  Travers,  was 
probably  the  best  example  of  the  possibilities  of  modern 
methods  in  the  detection  and  separation  of  minute  quantities 
of  new  elements  as  they  exist  in  Nature.  But  in  Marckwald's 
separations  the  proportion  of  the  new  substance  in  pitchblende 
is  certainly  less  than  this,  and  may  be  estimated  at  less  than 
one  five  hundred  millionth  part  of  the  whole.  We  shall  see  as 
we  proceed  that  the  minute  quantity  of  polonium  in  pitchblende 
is  intimately  connected  with  its  relatively  short-lived  activity 
.after  separation. 

Actinium. — Little  has  been  published  about  this  substance, 
.and  the  only  investigator  who  has  examined  it  is  Debierne, 
who  discovered  it  in  the  ammonium  hydrate  group  in  the 
.analysis  of  pitchblende  (Comptes  Rendus,  1899,  129,  p.  593; 
1900,  130,  p.  906 ;  1903,  136,  pp.  446  and  767).  The  element 
most  nearly  allied  to  actinium  in  its  analytical  behaviour  was 
first  stated  to  be  titanium  and  subsequently  thorium.  In  fact, 
the  preparations  were  shown  by  a  spectroscopic  examination 
by  M.  Demar9ay  to  consist  chiefly  of  thorium.  Like  radium, 


THE  RADIO-ACTIVE  ELEMENTS.  31 

it  gives  penetrating  rays  deviable  in  a  magnetic  field,  and  also 
a  characteristic  emanation  capable  of  imparting  temporary 
activity  to  objects  in  the  neighbourhood,  but  distinguished  from 
the  emanations  of  radium  and  thorium  by  a  more  rapid  rate  of 
decay  of  its  activity.  On  these  latterly  discovered  facts  its 
chief  claim  to  be  considered  a  new  radio-active  element  rests. 

Giesel  (Ber.,  1902,  p.  3,608;  or  Chemical  News,  1903, 
p.  97)  has  also  described  a  substance  which  appears  to 
be  identical  with  Debierne's  actinium.  He  calls  his  body 
"The  Emanation  Substance  from  Pitchblende,"  because  it 
is  characterised  by  great  power  of  giving  a  radio-active 
emanation.  Both  Debierne  and  Giesel  have  suggested  that 
the  new  body  is  the  cause  of  the  radio-activity  of  thorium, 
but  this  seems  impossible,  on  account  of  the  totally  distinct 
character  of  its  emanation.  It  resembles  thorium  and  the 
rare  earths  in  its  analytical  reactions,  being  precipitated 
from  its  solutions  by  hydrogen  peroxide  and  by  oxalic  acid. 
But  Giesel  has  obtained  it  free  from  thorium  and  nearly 
free  from  cerium  and  didymium,  and  finds  the  main  bulk 
of  the  substance  consists  of  lanthanum.  Lanthanum  is, 
however,  inactive,  and  stands  to  actinium  in  just  the  same 
relation  as  barium  to  radium,  or  bismuth  to  polonium,  as  the 
analytically  similar  element.  Giesel's  preparation  was  about 
one- thousandth  as  active  as  pure  radium  salts.  The  close 
resemblance  between  certain  very  curious  and  unique  properties 
of  the  emanation  from  both  Debierne's  actinium  and  Giesel's 
emanation-substance  (these  properties  being  discovered  in- 
dependently by  the  two  observers  for  their  respective  pre- 
parations) show  that  the  source  of  the  radio-activity  is  almost 
certainly  the  same  substance  in  the  two  cases.  These  special 
propeities  of  the  actinium  emanation  will  be  considered  later, 
after  the  other  emanations  have  been  studied  more  in  detail. 

The  Five  Radio-elements. — Five  radio-active  substances,  pos- 
sessing characteristic  radio-activity  are  thus  known — uranium, 
thorium,  radium,  polonium  and  actinium.  The  activity  of  the 
last  two  probably  only  persists  over  a  short  term  of  years, 
while  that  of  the  first  three  is  sensibly  permanent  at  least  for 
many  centuries.  These  bodies  present  many  peculiarities 
among  themselves  in  the  kind  of  radio-activity  they  exhibit 
(compare  Kutherford  and  Miss  Brooks,  Phil.  Mag.,  VI.  4,  p.  1, 


32 


RADIO-ACTIVITY. 


1902),  so  that  it  is  impossible  that  the  radio-activity  of  any 
two  or  more  of  them  can  be  caused  by  the  presence,  possibly 
in  minute  quantity,  of  the  same  substance. 

These  peculiarities  can  be  grouped  mainly  under  two  heads. 
The  first  is  the  character  of  the  rays  and  their  penetration- 
power.  Three  types  of  rays,  known  as  the  a,  ft  and  y  raysy 
arranged  in  the  ascending  order  of  their  power  to  penetrate 
matter,  are  recognised,  and,  although  the  rays  of  any  one  kind 
differ  slightly  among  themselves  in  penetrating  power  for 
different  radio-active  substances,  this  is  always  of  the  same 
order  for  the  same  type  of  ray  and  of  a  completely  different 
order  for  different  types  of  rays. 


Substance. 

o-rays. 

/3-rays. 

7-rays. 

Kadio-  active 
emanation. 

Imparts  activity 
to  surrounding 
objects. 

Uranium  .... 

Yes. 

Yes. 

Yes. 

No. 

No. 

Thorium  .... 

Yes. 
Yes. 
Yes. 

Yes. 

Yes. 

Yes. 

Yes. 

Eadium    .... 

Yes. 

Yes. 

Yes. 

Yes. 

Polonium    .  . 

No. 

No. 

No. 

No. 

Actinium  .... 

Yes. 

Yes. 

o 

Yes. 

Yes. 

The  second  distinctive  feature  which  serves  to  differentiate 
radio-active  substances  is  their  power  to  produce,  besides  rays, 
radio-active  gases,  not  necessarily  in  visible  quantities,  but 
possessing  sufficient  radio-activity  to  be  easily  detectable.  This 
property  was  discovered  for  thorium  by  Prof.  Rutherford, 
who  gave  the  name  "Emanation"  to  the  radio-active  gas  pro- 
duced from  thorium.  Uranium  and  polonium  do  not,  while 
thorium,  radium  and  actinium  do,  possess  this  property.  The 
respective  emanations  of  the  last  three  are  distinguished 
from  one  another  by  the  time  their  activity  lasts,  the  activity 
of  the  radium  emanation  lasting  several  weeks,  that  of  the 
thorium  emanation  only  a  few  minutes,  and  that  of  the 
actinium  emanation  a  few  seconds.  With  the  power  of 
a  radio-active  element  to  produce  a  radio-active  emanation 
is  bound  up  its  power  to  impart  radio-activity  to  objects 
in  the  neighbourhood.  Rutherford  discovered  this  property 
for  the  thorium  emanation,  and  designated  it  the  excited 


THE  RADIO-ACTIVE  ELEMENTS.  33 

activity.  M.  and  Mme.  Curie  discovered  it  simultaneously  for 
the  element  radium  (the  emanation  of  radium  was  not  then 
known),  and  called  it  the  induced  activity.  As  both  terms  are 
misleading  in  light  of  present  knowledge,  the  term  "  Imparted 
radio-activity "  will  generally  be  applied  in  this  book  to  this 
phenomenon.  In  the  preceding  table  a  list  is  given  of  the  radio- 
active substances  and  the  characteristics  of  their  radio-active 
properties  with  respect  to  the  features  just  enumerated. 

Although  radium  is  the  only  one  of  the  three  new  radio- 
elements  that  has  been  obtained  in  quantity  sufficient  for  its 
ordinary  material  properties  to  be  investigated,  the  strongest 
evidence  in  favour  of  considering  it  a  new  element  is  the 
unique  and  specific  character  of  its  radio-active  properties. 
The  emanation  of  radium  in  the  rate  of  decay  of  its  activity 
and  its  power  of  imparting  a  peculiar  sort  of  temporary  activity 
to  bodies  in  contact  with  it  is  a  far  finer  test  for  the  presence 
of  radium  than  the  spectrum  of  the  latter  element.  On 
this  account  it  is  certain  that  the  activity  of  the  other  four 
radio-elements  is  not,  for  example,  due  to  the  presence  of 
radium.  Conversely,  it  appears  likely  that  certain  supposed 
new  radio  elements,  as,  for  example,  the  so-called  radio-lead, 
consisted  merely  of  inactive  matter  admixed  with  a  trace 
of  radium.  The  radio-active  gas  that  has  been  described  from 
metallic  mercury,  from  soils  and  water,  &c.,  has  the  character- 
istics of  the  radium  emanation,  and  owes  its  origin,  in  all 
probability,  to  the  same  cause.  On  the  other  hand,  a  new  type 
of  emanation,  for  example,  indicates  the  presence  of  a  new 
radio-element,  and  many  such  will,  no  doubt,  be  discovered  by 
this  test  in  the  future. 

Source  of  Radio-active  Energy, — The  fundamental  problem 
presented  by  the  property  of  radio-activity  is  the  same  for  all  the 
radio-elements,  whether  they  exhibit  it  to  a  feeble  degree,  like 
uranium  and  thorium,  or  to  an  intense  degree,  like  radium.  In 
the  latter  case  the  magnitude  of  the  effects  render  the  nature  of 
the  problem  more  obvious,  but  the  difficulty  in  the  way  of  an 
explanation  is  no  greater  on  that  account.  To  cause  substances 
to  fluoresce,  to  ionise  a  gas,  or  even  to  fog  a  photographic  plate 
requires  energy.  In  the  case  of  the  production  of  cathode-rays 
and  X-rays  from  a  Crookes  tube,  the  source  of  the  energy  is,  of 
course,  to  be  found  in  the  electrical  forces  employed.  But  in 


34  RADIO-ACTIVITY. 

the  case  of  the  radio-elements  the  source  of  the  energy  is  not 
apparent.  The  emission  of  energy  from  radium,  for  example, 
is  at  once  spontaneous  and  persistent.  If  in  one  hour 
sufficient  heat  is  given  out  by  this  element  to  raise  its  own 
weight  of  water  from  the  freezing  to  the  boiling  point,  it  is  easy 
to  see  that  the  quantities  of  energy  liberated  in  a  year,  or  in  a 
thousand  years,  must  make  an  enormous  total.  In  only  two 
days  radium  gives  out  more  energy,  weight  for  weight,  than  the 
most  powerful  explosive  known  liberates  during  its  explosion. 
If  the  energy  liberated  in  a  thousand  }Tears  could  be  released 
instantaneously,  a  single  milligramme  of  radium  would  equal  in 
its  effect  a  ton  of  any  known  explosive.  There  are  only  two 
general  explanations  of  the  source  of  this  energy  possible, 
Jttither  radium  must  possess  the  power  of  responding  to  some 
hitherto  unknown  and  unsuspected  source  of  external  energy 
in  such  a  way  as  to  convert  the  latter  into  forms  which  come 
within  our  powers  of  recognition,  or,  the  energy  must  be 
derived  from  some  hitherto  untapped  internal  store  bound  up 
and  latent  in  the  structure  of  the  atom.  The  arguments  at 
the  present  time  are  overwhelmingly  in  favour  of  the  latter 
view.  Both  views  necessitate  very  substantial  additions  to- 
our  accepted  ideas.  But  whereas  the  extension  required  by 
the  latter  view  of  the  existence  of  forces  within  the  atom — 
large  compared  with  ordinary  chemical  or  molecular  forces — 
is  in  accordance  with  recent  advances  that  have  been  made  in 
our  knowledge  of  atomic  structure,  the  extension  required  by 
the  first  view  of  unknown  forms  of  energy  in  external  space 
is  wholly  new,  and  much  more  sweeping  in  its  consequences. 
For,  to  explain  the  energy  emitted  in  one  place  by  radium, 
all  space,  it  would  seem,  must,  equally  with  the  place  where 
the  phenomenon  is  manifested,  contain  a  similar  store  of 
potential  energy,  and  the  total  amount  of  existing  energy  in, 
the  universe  postulated  by  the  latter  view  is  far  greater  than 
in  the  former  case.  Moreover,  it  only  explains  the  source  of 
the  energy,  and  leaves  unanswered  how  it  is  that  it  is  mani- 
fested in  those  particular  ways — the  emission  of  rays,  the 
production  of  emanations,  &c. — which  comprise  all  the  complex 
manifestations  of  radio-activity.  On  this  view  also,  an  element 
once  radio-active  should  continue  so  indefinitely  without  change 
of  activity  or  loss  of  substance.  On  the  alternative  view  the- 


THE  RADIO-ACTIVE  ELEMENTS.  35 

opposite  holds  true.  The  element  must  be  undergoing  change, 
and  in  the  exhaustion  of  its  large,  but  still  not  infinitely  large, 
store  of  energy  the  element  must  itself  change  into  other 
forms  which  possess  a  smaller  energy-content.  It  is  the  main 
object  of  the  present  book  to  show  that  the  radio-elements 
are  undergoing  such  changes,  and  that  these  changes  are  new, 
in  that  they  involve  the  disappearance  of  the  atom  of  the 
element  and  its  transformation  into  other  atoms.  In  this 
way  it  will  be  shown  that  not  only  is  the  energy-emission 
explained,  but  also  that  all  the  complex  and  varied  phenomena 
exhibited  by  the  radio-elements  are  simple  and  necessary  con- 
sequences of  the  one  fundamental  conception. 


CHAPTER  III. 


THE  ELECTRICAL  PROPERTIES  OF  GASES. 

The  Ions  of  Gaseous  Conduction. — Distinction  beticeen  the  long  and  the 
Radio-active  Emanations. — The  Saturation  Current. — Equation  of 
Current  flowing  throuch  a  Gas. — Ionic  Velocities. — Coefficients  of 
Diffusion. — Determination  of  tlie  Cliarge  carried  by  an  Ion. — The 
"  Atomic  Charge." — C.  T.  E.  Wilson's  Condensation  Experiments. — 
Determination  of  the  Number  of  Molecules  in  a  Cubic  Centimetre  of 
Hydrogen. — Radiant  long. — Their  potcer  of  Ionising  Gases. — Stria. — 
Determination  of  the  Velocity  and  of  the  Ratio  of  the  Charge  to  the 
Mass  of  the  Radiant  Ion. — Direct  Determination  of  Velocity. — The 
Negative  Ions  produced  by  Metals  under  the  action  of  Ultra-violet 
Light. — The  Mass  of  the  Negative  Ion  or  Corpuscle. — Positive-Rays. — 
Electrical  Inertia  or  Mass. —  Variation  of  Electrical  Mass  with  the 
Velocity  of  the  Corpuscle  up  to  the  Speeds  of  Light. — The  Electronic 
Constitution  of  Matter. 

The  measurement  of  radio-activity  in  an  accurate  and  quan- 
titative manner  depends  upon  the  property  possessed  by  the 
rays  emitted  of  ionising  the  gas  through  which  they  pass—  i.e., 
of  rendering  it  capable  of  carrying  limited  quantities  of  both 
positive  and  negative  electricity.  This  was  shown  for  the 
uranium  rays  very  early  by  Becquerel.  The  use  of  this  pro- 
perty as  a  means  of  quantitative  measurement  dates,  however, 
from  the  work  of  Rutherford  (Phil.  Mag.,  1899,  V.,  50,  p.  109), 
who  showed  that  the  ionisation  produced  by  the  uranium  radia- 
tion is  of  the  same  character  as  that  produced  by  the  X-rays, 
and  equally  with  the  latter  can  be  simply  and  directly  explained 
by  the  theory  put  forward  by  Prof.  J.  J.  Thomson  that  the 
conduction  is  brought  about  by  the  transport  of  negatively  and 
positively  charged  carriers  to  the  positive  and  negative  elec- 
trodes respectively.  The  function  of  the  rays,  both  the  X-rays 


38  RADIO-ACTIVITY. 

and  the  rays  from  radio-active  substances,  is  to  produce  these 
ions  throughout  the  volume  of  the  gas.  The  energy  of  the 
rays  absorbed  by  the  gas  is  utilised  in  the  production  of  ions, 
so  that  the  number  produced  in  unit  time  is  a  measure  of  the 
intensity  of  the  radiation  absorbed.  The  current  flowing 
through  a  gas  under  certain  special  conditions  will  be  shown 
later  to  be  a  measure  of  the  number  of  ions  produced  per 
second.  Hence  it  becomes  possible  to  measure  radio-activity 
by  electrical  means.  The  actual  methods  employed  will  be 
dealt  with  in  the  next  chapter.  It  is  first  necessary  to  obtain  an 
idea  of  the  nature  of  the  ions  and  their  behaviour  under  given 
conditions. 

It  will  be  convenient  to  consider  first  the  ions  of  normal 
gaseous  conduction  which  are  produced  in  a  gas  whenever  it 
is  acted  on  by  certain  types  of  radiation  of  which  the  X-rays, 
the  cathode  or  Lenard  rays,  and  Becquerel  rays  are  the  most 
important,  but  which  we  now  know  are  produced  also  by 
ultra-violet  light  of  excessively  short  wave-length.  Ordinary 
air  screened  from  extraneous  influences  is  in  all  probability 
an  absolute  non-conductor  of  electricity.  Experimentally, 
it  always  exhibits  an  excessively  minute  conductivity,  which 
is,  however,  caused  by  ions,  and  generally  spoken  of  as 
the  natural  or,  less  correctly,  the  spontaneous  ionisation 
of  the  air.  But  since  this  continuously  decreases  as  greater 
precautions  are  taken  to  screen  it  from  the  action  of  infini- 
tesimal quantities  of  radio-active  matter,  it  is  a  fair  inference 
to  regard  the  air  as  a  perfect  insulator  when  in  its  normal 
condition.  If  it  is  got  into  a  conducting  state  by  the  action  of 
X-rays,  for  example,  it  is  found  that  its  conductivity  persists 
for  a  short  time  after  the  rays  are  cut  off,  so  that  the  con- 
ducting air  can  be  blown  away  from  the  neighbourhood  of  the 
X-ray  tube,  and  will  discharge  an  electrified  body,  as,  for 
example,  an  electroscope,  at  some  distance  out  of  the  direct  line 
of  fire  of  the  rays.  Fig.  5  represents  an  arrangement  for 
showing  this.  The  air  under  the  influence  of  the  X-rays  is 
drawn  by  a  pump  from  B  through  wide  tubes  into  the  electro- 
scope A  and  discharges  it.  If,  however,  the  air  is  passed 
through  a  filter,  like  cotton  or  glass  wool,  or  bubbled  through 
liquids,  or  exposed  in  its  passage  to  the  action  of  an  electric 
field,  it  ceases  to  conduct.  Thus  if  (Fig.  5)  an  electric  field 


THE  ELECTRICAL  PROPERTIES  OF  GASES. 


39 


is  made  to  operate  on  the  gas  in  its  passage  by  connecting  the 
outside  of  the  metal  tuhe  C  with  one  pole  of  a  battery,  the 
other  pole  being  connected  with  a  central  wire  insulated  and 
stretched  along  the  tube,  the  air  passing  into  the  electroscope 
no  longer  discharges  it.  Something  is  removed  out  of  the  air 
by  these  means  which  is  different  from  the  main  bulk  of  the  air, 
and  this  something  consists  of  the  charged  ions  which  carry 
the  electricity.  Since  the  gas  as  a  whole  shows  no  charge, 
it  follows  that  an  equal  number  of  positive  and  negative 
ions  exist  in  the  gas,  for  it  has  been  shown  that  the  charge 
carried  by  the  negative  ion  is  equal  to  that  carried  by  the 
positive. 

It  is  necessary  here  to  call  attention  to  the  distinction 
between  the  ions  and  the  radio-active  emanations  produced 
by  radium  and  thorium.  The  latter  also  possess  the  power 


Pump 


FIG.  5. 

of  being  blown  through  tubes  and  discharging  an  electroscope 
at  some  distance  from  the  radio-active  substance.  The 
emanations,  unlike  the  ions,  pass  unchanged  through  filters 
and  liquids  which  destroy  the  ions,  and  they  survive  the  action 
of  the  electric  field.  The  reason  why  the  electroscope  is  dis- 
charged in  the  latter  case  is  that  the  emanation  is  an  ionising 
agency,  and  continually  produces  fresh  ions  out  of  the  gas  after 
its  passage  through  the  plug.  The  emanation  passes  through 
the  latter  like  an  ordinary  gas,  and  immediately  produces 
fresh  ions  out  of  the  gas  on  the  further  side. 

The  most  characteristic  feature  of  the  conductivity  of  an 
ionised  gas,  by  which  it  is  easily  distinguished  from  all  other 
types  of  electrical  conduction,  is  the  existence  of  a  maximum 
or  "  saturation  "  current,  which  limits  the  quantity  of  electricity 


40  RADIO-ACTIVITY. 

the  gas  is  able  to  convey.  In  electrolytic  and  metallic 
conduction  the  current  obeys  Ohm's  law,  and  increases  pro- 
portionally to  the  voltage  between  the  electrodes.  This  is 
true  also  of  ionised  gases  under  very  small  voltages.  As  the 
latter  is  increased,  the  current  soon  begins  to  increase  less 
rapidly  than  the  voltage,  until,  finally,  a  stage  is  reached  at 
which  a  further  very  great  increase  of  voltage  scarcely  increases 
the  current  that  passes.  This  is  the  saturation  current,  and  it 
serves  experimentally  as  a  very  valuable  criterion  of  a  true 
ionisation  current.  Fig.  6  shows  the  variation  of  the  ionisa- 
tion  current  with  voltage. 

In  the  electrical  measurements  of  radio-activity,  it  is  always 
necessary  to  make  sure  either  (1)  that  the  current  measured  is 
the  saturation  current,  or  (2)  less  commonly,  and  with  very 
active  substances,  that  the  current  is  proportional  to  the  E.M.F., 


Voltage. 
FIG.  G. 

in  order  that  the  results  should  possess  a  definite  meaning, 
and  be  comparable  among  themselves.  The  ions  move  to  the 
electrodes  through  the  gas  under  the  influence  of  the  electric 
field.  They  obey  in  this  respect  the  ordinary  law  of  a  body 
moving  through  a  viscous  medium,  and  their  velocity  over 
the  ordinary  experimental  range  may  be  taken  as  being 
directly  proportional  to  the  force  acting  on  them — i.e.,  to  the 
strength  of  the  electric  field.  The  number  arriving  at  the 
electrodes,  and,  therefore,  the  strength  of  the  current  flowing, 
will  depend  (1)  on  the  total  number  present  in  the  gas  and 
(2)  on  their  velocity.  With  a  definite  rate  of  reproduction  of 
ions  by  the  ionising  agency  the  current  should,  if  there  were 
no  disturbing  factor  to  be  considered,  be  independent  of  the 
strength  of  the  electric  field — i.e.,  of  the  voltage.  The  product 


THE  ELECTRICAL   PROPERTIES   OF  GASES.  41 

of  the  number  present  and  their  velocity  would  be  constant, 
for  as  the  latter  decreases  the  number  accumulating  in  the  gas 
under  the  steady  rate  of  reproduction  will  correspondingly 
increase.  The  disturbing  factor  is  that  oppositely  charged 
ions  attract  each  other  and  tend  to  recombine  to  form  the 
neutral  molecule.  With  high  voltages  acting  this  influence 
will  be  reduced  to  a  minimum  and  the  saturation  current  will 
be  obtained.  This  represents  the  condition  in  which  the  ions 
in  the  gas  are  removed  by  passage  to  the  electrodes,  as  fast  as 
they  are  produced  by  the  ionising  agency,  and  before  any 
appreciable  proportion  recombine.  When  this  condition  is 
substantially  realised  the  current  will  be  independent  of 
further  increase  of  voltage.  Let  q  positive  and  q  negative 
ions  be  produced  per  second,  and  let  e  be  the  charge  carried 
by  the  individual  ion.  If  i  is  the  current  passing  through  the 
gas,  i  represents  the  value  of  the  charge  received  by  each  electrode 
per  second,  i/e,  therefore,  is  the  number  of  ions  which  give  up 
their  charge  to  either  electrode  per  second,  so  that  i/e  positive 
and  i/e  negative  ions  are  withdrawn  from  the  gas  per  second. 
The  number  withdrawn  by  the  saturation  current  represents 
the  number  produced.  Therefore,  i/e  =  q  and  i  =  qe  when 
current  is  the  maximum.  If,  as  is  usually  assumed  to  be  the 
case,  the  ions  are  produced  uniformly  throughout  the  volume  of 
the  gas,  and  qQ  represents  the  number  of  ions  produced  in  each 
cubic  centimetre  per  second,  and  if  A  is  the  area  of  the  elec- 
trodes and  /  the  distance  between  them,  q  =  q0A.lj  and  the 
saturation  current  through  the  gas  I  =  qQA.le. 

With  currents  less  than  the  saturation  current,  some  of  the 
ions  are  destroyed  in  other  ways  than  by  passage  to  the  elec- 
trodes. Some  will  re-combine,  one  positive  and  one  negative 
ion  reproducing  the  neutral  molecule,  while  others  will  give  up 
their  charge  by  diffusion  to  the  walls  of  the  containing  vessel 
and  so  cease  to  exist  as  such.  The  number  that  re-combine 
will  be  proportional  to  the  number  of  collisions  between  ions 
of  opposite  sign  and,  therefore  to  WjW,,  where  n^  and  n.2 
represent  the  number  of  positive  and  negative  ions  respec- 
tively present  in  1  cubic  cm.  of  gas.  Since,  in  the  cases 
generally  considered,  7^  =  w2,  the  number  that  re-combine  will 
be  proportional  to  n~,  and  may  be  expressed  by  an*,  where  a 
is  a  quantity  independent  of  n.  The  number  that  give  up 


42  RADIO-ACTIVITY. 

their  charge  to  the  walls  of  the  containing  vessel  is  propor- 
tional to  n  and  not  to  n2,  and  may  be  expressed  by  KTI,  where 
K  is  a  coefficient  depending  on  the  size  and  shape  of  the 
vessel. 

If  i  is  made  to  represent  the  current  flowing  through  unit 
area  of  the  gas,  i/le  represents  the  number  of  ions  of  each 
kind  withdrawn  by  the  current  for  unit  volume  of  gas.  So 
that,  when  the  gas  is  in  a  steady  state,  the  number  of  ions 
withdrawn  being  equal  to  the  number  produced, 

qQ  =  an2  +  KH  +  —  . 

v(s 

If  the  number  of  ions  present  in  the  gas  has  not  reached 
a  steady  state, 

dn  i 


This  is  the  simplest  general  equation  representing  the  current 
flowing  through  a  gas  ionised  by  Becquerel  or  X-rays.  The 
meaning  of  the  saturation  current  is,  thus,  the  current  that  is 
able  to  flow  through  the  gas  when  the  ions  are  withdrawn 
as  quickly  as  produced  by  the  action  of  a  strong  E.M.F. 
before  any  appreciable  number  have  time  to  be  dissipated  by 
recombination  or  by  diffusion  to  the  walls  of  the  containing 
vessel. 

With  no  current  flowing,  g0  =  an2  +  KU.  Except  for  narrow 
tubes,  the  number  lost  by  diffusion  to  the  walls  is  small 
compared  to  the  number  lost  by  recombination,  and,  con- 
sequently, it  is  often  unnecessary  to  take  the  quantity  Kn 
into  account.  With  regard  to  the  quantity  a?i2,  Langevin 
has  recently  shown  ("  Kecherches  sur  les  gaz  ionises,"  Univer- 
sity of  Paris,  1902)  that  recombination  is  brought  about  by  the 
mutual  attraction  of  the  positive  and  negative  ions,  for  if  it 
were  not  for  this  attraction  the  rate  of  recombination  would  be 
much  smaller  than  it  actually  is.  For  further  details  Prof.  J.  J. 
Thomson's  "  Conduction  of  Electricity  through  Gases,"  p.  545, 
should  be  consulted.  The  simple  statement  of  the  ionisation 
theory  here  given,  and  the  account  that  follows,  is  taken  from 
that  work. 

The  rate  at  which  the  ions  move  in  an  electric  field  was  first 
measured  by  Rutherford  (Phil.  Mag.,  1897,  V.,  46,  p.  422),  and 


THE  ELECTRICAL   PROPERTIES   OF  GASES. 


43 


was  found  to  be  comparatively  slow.  Zeleny  subsequently 
showed  that  the  negative  ions  move  appreciably  faster  than  the 
positive  ions,  and  the  ratio  of  the  two  velocities  varies  very  much 
with  different  gases  and  with  their  state  of  dryness.  Water 
vapour  tends  to  equalise  the  velocities  by  diminishing  that  of  the 
negative  ion.  The  following  table  of  ionic  velocities,  taken 
from  Prof.  Thomson's  book,  expresses  the  results  obtained  by 
Zeleny  (Phil  Trans.,  A.  195,  p.  193,  1900).  The  ionisation 
was  produced  by  X-rays  : — 

Ionic  Velocities. 


Gas. 

Velocities  in  cm.  per  sec. 
under  a  potential  gradient 
of  1  volt  per  cm. 

Eatio  of 
velocities  of 
negative  and 
positive 
ions. 

Temperature. 
Degrees 
Centigrade. 

Positive  ions.  (Negative  ions. 

Air  drv.  . 

1-36 

1-87 

1-375 

13-5 

Air  moist  

1-37 

1-51 

1-10 

14-0 

Oxygen  dry  .... 
Oxygen  moist.. 
OOs  dry  .. 

1-36 
1-29 
0-76 

180 
1-52 
0-81 

1-32 
1-18 
1-07 

17-0 
160 
17-5 

C02  moist  
Hydrogen  dry  .  . 
Hydrogen  moist 

0-82 
6-70 
530 

0-75 
7-95 
560 

0-915 
1-19 
1-05 

17-0 
20-0 
20-0 

Townsend  has  also  measured  the  coefficients  of  diffusion  of 
the  ions  produced  by  various  agencies,  and  his  results  are 
given  in  the  following  tables.  The  X-rays  were  the  ionising 
agent  used : — 

Coefficients  of  Diffusion  in  Dry  Gases. 


Gas. 

Dfor  +  ions. 

D  for  -  ions. 

Mean  value 
of  D. 

Ratio  of  Dfor 
+  and  —  ions. 

Air    

0-028 

0-043 

0-0347 

1-54 

Oxvsen 

0-025 

G'0396 

0-0323 

1  58 

Carbon    dioxide 
Hydrogen     .... 

0-023 
0-123 

0-026 
0-190 

0-0245 
0-156 

1-13 
1-54 

Coefficients  of  Diffusion  in  Moist  Gases. 


Air     

0-032 

0-035 

0'0335 

1-09 

Oxygen 

0-0288 

0-0358 

0-0323 

1-24 

Carbon     dioxide 
Hydrogen     .... 

0-0245 
0-128 

0-0255 
0-142 

0-025 
0-135 

1-04 
1-11 

These  coefficients  of  diffusion  a  re  remarkably  slow  compared 
with  the  values  obtained  for  the  coefficients  of  diffusion  of  the 
uncharged  gas  molecules.  For  example,  the  value  for  the 


44  RADIO-ACTIVITY. 

positive  ion  in  air  (0'028)  is  less  than  the  value  of  a  heavy 
vapour  like  ether  or  isobutylamine  diffusing  into  carbon  dioxide 
(0-055  and  0-03  respectively). 

But  the  ions  show  the  same  coefficient  of  diffusion  and  the 
same  velocity  under  unit  electric  force  whatever  the  agency 
employed  to  produce  them.  The  negative  ion  diffuses  about 
half  again  as  fast  as  the  positive  in  dry  gases  like  oxygen 
or  hydrogen,  but  this  difference  is  much  diminished  by 
moisture.  The  diffusion  coefficient  D  >s  connected  simply 
with  the  velocity  u  of  the  ions  under  unit  electric  force, 
by  the  equation 


where  N  is  the  number  of  molecules  in  1  cubic  cm.  of  a  gas,  e 
the  charge  on  the  ion  and  II  the  pressure  of  the  gas  in  dynes  per 
square  centimetre.  It  is  thus  possible  to  determine  Ne,  and, 
since  N  is  known  from  the  kinetic  theory  of  gases  within 
certain  limits,  to  obtain  an  estimate  of  e,  the  charge  carried 
by  the  individual  ion.  But  the  quantity  NE,  where  E  is  the 
charge  carried  by  the  hydrogen  ion  in  electrolysis,  is  accu- 
rately known,  for  2NE  (since  one  molecule  of  hydrogen  con- 
tains two  atoms)  represents  the  number  of  electrostatic  units 
of  electricity  which  must  pass  through  acidulated  water  to 
liberate  1  cubic  cm.  of  hydrogen  as  gas.  Now  NE  =  1*22 
x  1010,  whereas  the  mean  of  the  values  of  Ne  obtained  by  the 
aid  of  the  above  equation  is  1-24  x  1010.  Hence  it  may  be 
inferred  that  the  charge  carried  by  the  positive  or  negative  ion 
in  gases  is  independent  of  the  nature  of  the  gas,  of  the  ionising 
agency  used  to  produce  it,  and  is  equal  to  the  charge  carried 
by  the  hydrogen  atom  in  electrolysis.  This  charge  will,  in 
future,  be  referred  to  as  the  "  atomic  charge." 

One  of  the  most  beautiful  and  striking  developments  of  the 
ionisation  theory  arises  out  of  a  discovery  by  C.  T.  R.  Wilson 
that,  if  dust-free  air,  free  from  ions  and  saturated  with 
water  vapour,  is  suddenly  expanded,  so  as  to  chill  it  and 
cause  supersaturation,  no  deposition  of.  moisture  occurs  pro- 
vided the  expansion  is  not  too  great.  If,  however,  positive  or 
negative  ions  are  present,  these  serve  as  nuclei  for  the  conden- 
sation, and  a  cloud  is  produced,  provided  the  expansion  is 
above  a  certain  limiting  value.  If  the  ratio  of  the  volumes, 


THE  ELECTRICAL  PROPERTIES   OF  GASES.  45 

after  and  before  expansion,  is  below  1*25,  no  deposition  of 
moisture  occurs.  For  the  ratio  1'25  up  to  1-3  only  the  nega- 
tive ions  serve  as  nuclei,  while  for  ratios  above  1-3  both  positive 
and  negative  ions  are  caught  and  carried  down  by  the  cloud. 
The  drops  are  so  small  that  the  cloud  takes  a  measurable  time 
to  settle  down  under  the  action  of  gravity,  so  that  the  rate  of 
fall  can  be  used  to  determine  the  size  of  the  water  drops.  If 
the  quantity  of  water  present  in  the  cloud  is  known,  the 
number  of  drops  can  be  determined,  and,  therefore,  the 
number  of  ions  which  served  as  the  nuclei  for  the  formation  of 
these  drops  can  actually  be  counted.  The  total  charge  carried 
by  the  ions  can  be  simply  determined  by  electrical  means,  and 
thus  a  direct  measure  can  be  obtained  of  the  charge  carried  by 
the  ion.  Prof.  Thomson  has  accomplished  this  result  for  the 
ions  produced  by  X-rays,  by  the  radium  rays,  and  for  the 
negative  ions  which  are  emitted  from  metals  under  the  influence 
of  ultra-violet  light,  and  he  has  proved  that  the  value  of  e  is 
the  same  for  all  ions,  positive  or  negative,  and  is  equal  to 
3-4  xlO'10  electrostatic  units  ((gr.)i(cm.)?(sec.)-1).  From 
NE  =  l-22xl010,  and  the  identity  of  E  and  e,  N  can  be 
directly  calculated  for  the  first  time  in  an  accurate  manner. 
The  value  thus  obtained  for  the  number  of  molecules  in 
1  cubic  cm.  of  hydrogen  gas  at  normal  temperature  and 
pressure  is  3*9  x  1019,  a  value  which  is  in  good  agreement  with 
the  estimates  based  on  the  kinetic  theory  of  gases.  For  fuller . 
particulars  and  for  the  experimental  methods  employed  to 
obtain  these  results  the  reader  is  again  referred  to  Prof. 
Thomson's  book.  (Compare  also  Sir  Oliver  Lodge :  "  On 
Electrons,"  The  Electrician,  Vols.  L.  and  LI,) 

Xo  direct  measurements  of  the  masses  of  the  ions  produced 
from  gases  have  yet  been  accomplished.  From  considerations 
connected  with  the  slow  rate  of  diffusion  and  of  movement  in 
an  electric  field  it  seems  probable  that  they  consist  of  aggre- 
gates of  several  molecules  of  the  gas,  perhaps  held  together 
by  the  attraction  of  the  field  due  to  the  charge  carried  by 
the  ion.  This  cluster  of  molecules  seems  to  be  smaller  for  the 
negative  ion  than  for  the  positive  in  dry  gases,  whereas  when 
moisture  or  vapours  like  alcohol  or  ether  are  present  they  tend 
to  condense  on  the  negative  ion  more  than  on  the  positive, 
and  so  diminish  its  freedom  of  movement. 


46  RADIO-ACTIVITY. 

Prof.  Thomson's  ionisation  theory  of  gaseous  conduction 
possesses,  however,  another  quite  independent  and  more  funda- 
mental bearing  upon  radio-activity. 

The  terms  "ion"  and  "ionisation"  appear  in  so  many  different 
connections  in  science  that  they  are  apt  to  be  somewhat  con- 
fusing. We  have,  for  example,  -the  ionisation  of  gases,  the 
ionic  theory  of  electrolytic  conduction,  and  the  ionic  dissocia- 
tion theory  of  solution.  The  fundamental  meaning  of  the 
word  ion  should  be  carefully  kept  in  view.  The  term  in  all 
these  cases  is  used  strictly  in  the  original  sense  of  Faraday — to 
express  a  moving  particle  carrying  an  electric  charge.  Thus 
Prof.  Thomson  speaks  in  his  book  of  the  positive  and  negative 
ions  expelled  by  radium  and  the  other  radio-elements,  alluding 
to  the  a  and  /3-rays.  The  a  and  /2-rays  consist  of  moving 
particles  carrying  an  electric  charge,  and,  strictly  speaking 
therefore,  are  ions. 

The  a-rays  are  positive  ions  and  the  /3-rays  negative  ions. 
There  is,  however,  this  distinction  between  the  two  classes. — 
The  ions  produced  in  gases  by  the  passage  of  the  X-rays  and 
similar  agents  from  radio-active  substances  are  characterised 
solely  by  their  charge.  Their  movement  in  an  electric  field  is 
the  result  of  the  charge  carried,  and  the  charge  is  the  only 
means  we  possess  of  detecting  and  investigating  them.  With 
the  projected  charged  particles  shot  out  by  radium,  the 
kinetic  energy  of  the  moving  body  and  not  its  charge  is  its 
main  experimental  characteristic.  In  consequence,  this  class  of 
ions  has  a  great  many  properties  not  possessed  by  the  ions  of  the 
first  class,  and  it  might  be  convenient  to  designate  it  by  the 
name  of  "  radiant  ion."  The  means  of  investigation  are  quite 
distinct  for  the  two  classes.  Yet  it  is  possible  to  start  with 
an  ion  of  the  first  class  and  impart  to  it  so  much  energy  that 
it  passes  into  the  second  class  and  resembles  in  properties  the 
radiant  ions  of  radium  and  uranium.  The  problems  of 
radio-activity  are  thus  intimately  bound  up  with  the  electrical 
properties  of  gases. 

We  have  seen  that  under  ordinary  circumstance  the  ions 
of  gaseous  conduction  travel  slowly  in  an  electric  field,  as  if 
hampered  by  a  cluster  of  molecules  attached  to  it.  A  remark- 
able change,  however,  comes  over  the  property  of  the  ion  when 
the  pressure  of  the  gas  is  sufficiently  reduced.  Its  movement 


THE  ELECTRICAL   PROPERTIES   OF  GASES. 


47 


then  appears  to  be  no  longer  hampered  by  the  condensation  of 
an  aggregate  of  molecules  around  it,  and  it  travels  free.  Under 
the  action  of  a  sufficiently  powerful  electric  force  it  then  acquires 
enormous  velocity,  and  the  increase  in  its  kinetic  energy  above 
a  certain  critical  value  causes  a  change  to  come  over  its  pro- 
perties ;  that  is,  the  ion  passes  from  the  first  class,  where  its 
charge  is  its  characteristic,  to  the  second  class,  where  its  energy 
begins  to  dominate  its  experimental  properties.  It  is  true  in 
the  case  just  considered  the  energy  is  the  result  of  the  action 
of  an  electric  force  on  the  charge  carried,  but  in  the  case  of 
radio-active  substances  projected  charged  particles  are  shot  out 
spontaneously  which  do  not  owe  their  velocity  or  energy  to  the 
action  of  any  external  force  upon  the  charge  they  carry.  The 
velocity  of  these  particles  is  far  higher  than  any  that  have 
been  obtained  bv  the  action  of  the  electric  field. 


Volt*. 
FIG.  7. 

The  most  characteristic  property  of  the  radiant  ions  above 
their  critical  velocity  is  their  power  to  ionise  the  gas  through 
which  they  pass.  This  has  been  particularly  studied  by  Town- 
send  (Phil.  Mag.,  1901,  VL,  1,  pp.  198  and  630).  If  the  voltage 
is  increased  far  beyond  that  required  to  produce  the  satura- 
tion current  in  a  gas  at  low  pressure,  it  is  found  that  above 
a  certain  point  the  current  again  commences  to  increase,  and 
does  so  with  great  rapidity  as  the  voltage  increases,  until  finally 
the  point  is  reached  at  which  a  spark  or  luminous  discharge 
passes  through  the  gas.  This  is  shown  in  Fig.  7,  which  would 
represent  the  relation  between  current  and  voltage  for  plates 
10cm.  apart  at  about  3mm.  pressure.  A  potential  gradient  of 
40  volts  per  centimetre  distance  between  the  electrodes  is 


48 


RADIO- A  CTI VI T  Y. 


sufficient  for  the  ions  to  acquire  sufficient  velocity  at  a  pressure 
of  1mm  to  start  ionising  the  gas.  It  can  be  shown  that  at  first 
it  is  only  the  negative  ion  which  acquires  this  power,  and  the 
number  of  ions  produced,  although  largely  increased,  still 
remains  finite.  For  the  negative  ions  all  travel  in  the  one  direc- 
tion, and  of  the  ions  they  produce  by  collision  with  the  gas  mole- 
cules, it  is  only  the  negative  ions  which  acquire  sufficient  velocity 
to  ionise  fresh  molecules.  Hence,  as  soon  as  these  ions  reach 
the  electrode,  the  action  is  at  an  end.  But  when  the  positive 
ion,  which  at  these  low  pressures  is  far  more  massive  than  the 
negative  ion,  reaches  the  critical  velocity  and  begins  to  pro- 
duce fresh  ions,  the  ions  travelling  in  both  directions  produce 
ions  in  their  path.  This  action  must  go  on  cumulatively,  for 
it  no  longer  ceases  when  the  ions  reach  the  electrode,  as  the 
ions  of  opposite  sign  are  always  travelling  anew  back  to  the 


70 


:40 

I  30 


10 


\ 

1 

\ 

So 

c 

*./" 

>  r 

.A/VYA 

\7 

s 

j  \ 

s 

No 

L 

>ativ 
ifiht 

'./"S 

% 

Pos  live 

W'l 

Column 

/->. 

^•s 

j^ 

/ 

01      23      45      6      7      8     9    10    11    12    13    14  CHI. 
Hydrogen  Pressure  %'25  mm.    Current  0'586  m.a. 
FIG.  8. 

-other  electrode  and  ionising  the  gas  in  their  path.  A  little 
consideration  will  show  that  the  number  of  ions  increases 
rapidly  to  an  almost  infinite  extent,  provided  the  electrical 
force  driving  the  ions  is  maintained  unaltered  in  value.  At 
this  point  a  spark  passes  through  the  gas.  Under  some 
•circumstances,  however,  the  sudden  increase  of  the  number 
of  the  ions  and  their  electrical  convection  tends  to  so 
diminish  the  force  of  the  effective  electric  field  that  this  is 
lowered  below  the  point  at  which  ions  are  produced  by 
collision.  It  is  probable  that  the  striated  discharge  in  vacuum 
tubes  is  produced  by  this  action,  as  the  distribution  of  electric 
force  through  the  tube  shows  a  rapid  alternation  in  intensity 
corresponding  with  the  position  of  the  strise.  This  is  shown 


THE  ELECTRICAL  PROPERTIES  OF  GASES.  49 

in  Fig.  8,  which  represents  the  electric  force  per  centimetre 
on  the  vertical  axis  for  different  parts  of  a  tube  showing 
striae  plotted  on  the  horizontal  axis  (compare  H.  A.  Wilson, 
Phil.  Mag.,  1900,  V.,  49,  p.  505).  The  discharge  thus  con- 
sists of  a  succession  of  periods  in  which  the  electric  force 
alternately  rises  above  and  sinks  below  the  critical  value 
necessary  for  the  ions  present  to  produce  other  ions  out  of  the 
gas.  The  well-known  experiment  of  Hittorf,  who  showed  that 
a  discharge  refused  to  pass  between  electrodes  1mm.  apart  in  a 
gas  at  low  pressure,  but  preferred  a  circuitous  route  375cm. 
long  (Fig.  9),  is  to  be  explained  by  similar  considerations. 
In  the  space  of  1mm.  the  ions  moving  under  the  electric  field 
cannot  acquire  the  critical  velocity  required  to  ionise  the  gas. 

An    independent    way   of    regarding    the    phenomena    of 
discharge  at  low  pressure  is  arrived  at  from  the  consideration 


FIG.  9. 

that  the  viscosity  of  the  gas  is  much  diminished.  In  conse- 
quence, the  ions  do  not  obey  the  law  followed  at  atmospheric 
pressure  that  their  velocity  is  proportional  to  the  force  acting. 
The  force  accelerates  the  unhampered  ion,  and  its  velocity  is, 
therefore,  greater,  the  greater  the  distance  traversed  under  the 
action  of  a  constant  force 

The  most  instructive  property  of  a  charged  particle  moving 
with  great  velocity  is  its  power  of  being  deviated  out  of  a 
straight  line  by  the  action  of  electric  and  of  magnetic  forces, 
since,  by  a  study  of  these  deviations,  we  can  arrive  at  the 
determination  of  the  velocity  of  the  particle  and  of  the  ratio 
elm  of  the  charge  of  the  particle  to  its  mass.  These  considera- 
tions are  of  great  importance  in  radio-activity,  for  by  aid  of 
them  knowledge  has  been  obtained  of  the  mass  and  velocity  of 
the  projected  particles  from  radio-active  substances. 


50  RADIO-ACTIVITY. 

If  the  charged  particle  is  projected  with  uniform  velocity  at 
right  angles  to  the  lines  of  magnetic  force,  no  electrical  forces 
acting,  its  path  is  changed  to  that  of  a  circle  whose  radius  of 
curvature,  /o,  is  given  by 


mv 


where  m  is  the  mass  of  the  ion,  e  its  charge,  v  its  velocity,  and 
H  the  magnetic  force.  This  enables  us  to  determine  the  value 
of  e/mv.  If  an  electrostatic  force  is  made  to  act  simultaneously 
on  the  moving  ion,  so  that  the  lines  of  electric  force  are  at 
right  angles  to  the  lines  of  magnetic  force,  and  at  right  angles 
also  to  the  direction  of  motion  of  the  ion,  the  electric  force 
can  be  made  either  to  neutralise  or  assist  the  deviation 
produced  by  the  magnetic  force  according  as  the  sign  of  the 
P.D.  is  in  the  one  direction  or  the  other.  If  it  is  adjusted 
so  as  to  oppose  the  magnetic  deviation,  a  point  can  be  reached, 


FIG.  10. 

by  the  suitable  variation  of  one  of  the  forces,  the  other 
remaining  constant,  at  which  the  projected  particle  is  not 
deviated.  At  this  point  it  can  be  shown  that  the  force  Fe, 
where  F  is  the  electric  force  and  e  the  charge  on  the  ion,  is 
balanced  by  the  force  Hev.  Fe  =  H^r.  Therefore,  v  =  F/H,  and 
the  velocity  of  the  ion  can  be  determined.  When  v  is  known 
the  ratio  e/m  can  also  be  determined  from  the  ratio  e/mv. 

In  this  way  Prof.  Thomson,  in  1897,  succeeded  in  determining 
the  value  of  e/m  and  of  v  for  the  negatively  charged  particle 
which  constitutes  the  cathode  ray  of  the  Crookes  tube.  The 
apparatus  used  is  shown  in  Fig.  10.  Cathode  rays  starting 
from  C  are  projected  through  the  diaphragms  AB,  which 
consist  of  fine  horizontal  slits  in  metal  discs :  they  then 
pass  between  the  two  horizontal  plates  ED,  maintained 
at  a  constant  P.D.,  and  fall  upon  the  phosphorescent  screen 


THE  ELECTRICAL  PROPERTIES   OF  GASES.  51 

at  the  end  of  the  tube.  The  vacuum  in  the  tube  must 
be  of  the  highest,  or  otherwise  the  ionised  gas,  being  a  con- 
ductor, screens  the  rays  from  the  action  of  the  electrostatic 
field  ED.  The  value  elmv  was  determined  by  measuring 
the  deviation  of  the  spot  of  light  on  the  screen  pp'  under 
the  action  of  a  known  magnetic  field.  An  electric  field  of 
known  intensity  is  then  applied  at  ED,  so  as  to  bring 
back  the  spot  of  light  to  its  original  position,  and  the 
value  of  v  and  of  e/'m  thus  found.  Prof.  Thomson  obtained 
/•  =  2'S  x  109  cm.  per  second  and  e/m  =  7'7  x  106  as  the  mean 
result  of  his  determinations.  The  velocity  of  light  is  about 
3  x  1010  cm.  per  second,  so  that  the  cathode  rays  consist  of 
streams  of  charged  particles  travelling  with  one-tenth  of  the 
velocity  of  light.  From  the  direction  of  the  deviation  it  follows 
that  the  particles  must  be  negatively  charged.  The  value  e/m 
for  the  hydrogen  ion  in  electrolysis  is  only  104,  so  that  the 
value  for  the  cathode  ray  particle  is  770  times  greater  than 
for  the  hydrogen  ion.  This  value  was  found  to  be  inde- 
pendent of  the  nature  of  the  gas  in  the  tube,  although  a  wide 
range  of  gases,  represented  by  the  extremes  of  hydrogen 
and  methyl  iodide,  was  experimented  on,  and  it  has  been 
proved  to  be  the  same  whatever  the  nature  of  the  metal  of 
which  the  cathode  is  made.  Lenard  investigated  the  values 
of  v  and  e/m  for  the  cathode  rays  from  an  exhausted  tube  that 
escaped  through  a  window  of  very  thin  aluminium  foil,  and 
found  that  while  e/m  was  in  good  agreement  with  Thomson's 
value  the  value  of  v  was  much  higher — viz.,  7  x  10!),  indicating 
that  the  aluminium  foil  had  sorted  out  the  more  rapidly 
moving  particles,  but  that  the  value  ejm  was  independent  of 
the  velocity. 

The  theoretical  reasoning  on  which  these  determinations  are 
based  has  been  confirmed  in  the  most  brilliant  manner  by  the 
direct  determination  of  v  by  Wiechert  (Wied.  Ann.  1899,  p. 
739).  The  principle  of  the  method  was  to  send  a  Leyden-jar 
discharge  through  two  circuits  at  different  positions  along  the 
tube  through  which  the  cathode  rays  passed.  The  Leyden- 
jar  discharge  consists  of  very  rapidly  alternating  currents 
whose  frequency  can  be  accurately  determined.  If  the  distance 
between  the  two  circuits  is  so  arranged  that  the  cathode 
ray  takes  the  time  of  a  complete  oscillation  to  traverse  it, 

E2 


52  RA  DIO-A  GTIVIT  Y. 

the  deflection  produced  by  the  first  circuit  will  be  augmented 
by  the  second  circuit.  If,  however,  the  distance  is  so  arranged 
that  by  the  time  the  cathode  ray  leaves  the  first  circuit  to 
the  time  it  reaches  the  second  circuit  only  half  a  period  of 
oscillation  has  elapsed,  then  the  deflections  produced  will 
neutralise  one  another,  and  the  path  of  the  ray  will  appear 
straight.  The  value  of  v  found  by  Wiechert  in  this  way  was, 
for  one  experiment,  5  x  109.  From  the  magnetic  deviation 
of  the  same  rays  the  value  mv/e  was  determined,  and  combin- 
ing this  with  the  direct  experimental  value  of  v  the  value 
e/m  =  from  1'5  to  1'OxlO7  was  obtained.  Considering  the 
difficulty  of  the  experiments,  this  value  is  in  good  agreement 
with  the  others,  and  supports  the  general  argument  in  the 
strongest  possible  manner. 

The   connection  between   the    negative   ion    produced   by 
X-rays,  &c.,  and  the  negative  radiant  ion  which  constitutes  the 
cathode  particle  is  shown  by  the  following  experiments  : — If 
ordinary  ultra-violet  light  (not  of  the  extreme  wave-lengths 
before  mentioned,  which  are  able  to  ionise  the  air  like  X-rays 
and  to  produce  both  +  and  —  ions)  is  allowed  to  fall  on  a 
polished  surface  of  zinc,  the  latter  expels  negative  ions  only 
into  the  gas.     If  the  zinc  is  negatively  charged  it  rapidly 
loses   its   charge   in  ultra-violet  light,   but   remains   charged 
indefinitely  when  its  electrification  is  positive.     If  the  experi- 
ment is  repeated  in  air  at  low  pressure,  the  negative  ions 
produced  by  the  zinc  acquire  in  an  electric  field  the  character- 
istics of  the  cathode-ray  particle,  and  are  shot  out  normally 
to   its   surface.     The   deviation   produced    by   magnetic  and 
electric  forces  can  be  measured  as  in  the  case  of  the  cathode 
rays.     The  values  of  v  and  of  e/m  for  these  ions  have  been 
determined  by  Prof.  Thomson  (Phil.  Mag.,  1899,  p.  547)  and 
by  Lenard  (Drudes  Annalen,  1900,  p.  359).    The  former  found 
for  e/m  the  value  7'3  x  106,  which  is  in  very  good  agreement 
with  the  value  for  the  cathode-ray  particle — viz.,  7'7  x  106. 
These    negative    ions    have    been    very    carefully    studied. 
Produced    in    a    gas    at    atmospheric    pressure,    they   have 
been  shown  to  be  identical  with  the  negative  ions  produced 
by   other   agencies    (as,    for   example,   the    X-rays)   in   their 
coefficient  of  diffusion  and  their  velocity  under  unit  potential 
gradient  and,  therefore,  in  the  value  of  e,  the  charge  carried. 


THE  ELECTRICAL  PROPERTIES  OF  OASES.     53 

Moreover,  a  direct  determination  of  the  value  of  e  has 
been  accomplished  by  Prof.  Thomson  by  the  condensation 
method,  and  found  to  be  the  same  as  that  carried  by  the 
hydrogen  ion  in  electrolysis.  Yet  this  same  ion,  when  the 
air  is  removed  and  it  travels  free  from  entanglement,  then 
resembles  the  cathode  ray  in  all  its  properties,  and  is  found  to 
possess  a  value  of  elm  700  times  greater  than  that  possessed 
by  the  hydrogen  ion  in  electrolysis.  If  the  charge  on  the 
ion  is  assumed  to  be  invariably  the  "  atomic  charge " 
possessed  in  gases  at  ordinary  pressure,  the  conclusion  is 
reached  that  the  negative  ion,  when  it  travels  free,  possesses  a 
mass  y^otn  °*  tne  mass  of  tne  hydrogen  atom.  Since  the  value 
of  e/m  agrees  for  all  negative  ions  for  which  it  has  been  deter- 
mined, it  follows  that  this  ion  possesses  an  invariable  mass 
about  y^oth  of  that  possessed  by  the  lightest  atom  known, 
and  that  negative  electrification  in  high  vacua,  when  but  little 
ordinary  matter  is  present,  consists  of  an  assemblage  of  discrete 
charged  "corpuscles"  of  far  smaller  mass  than  anything  before 
observed. 

The  presence  of  positive  ions  in  the  electric  discharge  in  vacuo 
is  manifested  in  a  type  of  radiation  discovered  by  Goldstein, 
called  by  him  "  Canalstrahlen  "  (Canal  Rays),  but  which  may 
be  termed  the  positive  rays.  They  are  deflected  by  electric  and 
magnetic  fields  in  the  opposite  sense  to  the  deflection  produced 
in  the  path  of  the  cathode  rays,  but  only  with  very  much 
greater  difficulty.  Wein  showed  the  value  of  e\m  for  these  rays 
to  be  3  x  102,  while  v  =  3*6  x  10'cm.  per  second.  The  subject  has 
since  been  investigated  by  J.  J.  Thomson  for  the  positive  ions 
emitted  from  heated  wires  in  vacuo,  and  he  found  a  similar 
value,  e/w  =  4x!02.  Thus  this  value  is  only  about  30oootn 
part  of  that  shown  by  the  negative  ion,  and  is  of  the  same 
order  as  in  the  case  of  electrolysis  for  heavy  ions  like  those 
of  oxygen  or  the  metals.  Since  the  value  e/m  of  the  posi- 
tive ion  has  never  been  observed  to  be  greater  than  that 
observed  for  the  hydrogen  ion  in  electrolysis,  it  follows  that 
the  positive  ion,  unlike  the  negative,  is  never  dissociated  from 
the  atom  which  carries  it.  On  this  view  Prof.  Thomson  has 
developed  the  "  corpuscular  theory  of  electricity  "  which  sup- 
poses that  the  negative  charges  are  discrete  particles  or 
corpuscles  (called  electrons  by  other  investigators),  and  that  the 


54  RADIO-ACTIVITY. 

absence  of  one  of  these  corpuscles  from  the  atom  of  matter 
produces  the  positive  ion. 

It  has  been  shown  mathematically  by  Thomson  and  Heavi- 
side  that  an  electric  charge  concentrated  on  a  sphere  of 
sufficiently  minute  radius  would  possess  inertia  by  virtue  of 
the  electromagnetic  field  of  force  it  creates  in  the  surrounding- 
ether.  That  is  to  say,  it  will  tend  to  resist  change  of  motion, 
and  in  general  will  behave  as  if  possessing  a  certain  mass. 
Electricity  will,  therefore,  under  these  conditions,  resemble 
matter  in  its  most  fundamental  property.  One  of  the  con- 
sequences deduced  from  this  view  is  that  the  speed  of  the 
ion  cannot  be  increased  beyond  a  certain  limited  value,  repre- 
sented by  the  velocity  of  light,  because  at  this  value  the  inertia, 
and  therefore  the  mass,  of  the  particle  would  be  infinite.  For 
values  of  v  below  one-half  the  velocity  of  light  the  inertia  due 
to  the  moving  charge,  or  as  it  is  convenient  to  call  it,  the  elec- 
trical mass,  is  approximately  constant,  hut  above  that  speed  the 
mass  increases  rapidly.  Sir  Oliver  Lodge  (Nature,  June  11, 
1903,  p.  129)  has  given  some  calculations  of  the  ratio  of  the 
electrical  mass  mQ  for  slow  motions  to  the  mass  m  when  the 
velocity  of  light  is  approached.  At  half  the  speed  of  light 
m  =  l-12w0;  at  three-quarters  m^l'37w0;  at  nine-tenths 
m  =  l'Sm0',  when  the  speed  is  99  per  cent,  of  that  of  light 
w=3-2Sm0,  for  99'9  per  cent.  m  =  5m0,  whereas  between  this 
last  value  and  that  of  light  the  mass  increases  to  infinity.  It 
is  to  be  remarked  how  nearly  the  velocity  of  light  must  be 
approached  before  the  action  becomes  pronounced.  One  of 
the  most  remarkable  facts  of  radio-activity  has  been  the  dis- 
covery that  radium  projects  negative  corpuscles,  or  /3-rays,  with 
speeds  varying  within  the  limits  between  which  an  increase 
of  mass  is  to  be  theoretically  expected,  if  this  mass  is  electrical 
in  origin.  This  increase  has  been  looked  for  and  found  by 
Kaufmanri  and  his  work  will  be  considered  when  the  rays 
from  radium  are  studied. 

It  will  be  Seen  that  the  tendency  of  these  developments  has 
been  to  replace  the  attributes  of  matter  by  those  of  electricity, 
and  to  look  up~>n  the  atom  as  a  very  complex  system  made  up 
of  much  smaller  units  of  electricity,  or  corpuscles.  If  one 
excludes  the  minute  and  unexplained  effect  of  gravitation, 
which  is  only  so  prominent  to  us  on  account  of  the  nearness  of 


THE  ELECTRICAL  PROPERTIES  OF  GASES.  55 

the  overwhelming  mass  of  the  earth,  the  main  attribute  of 
matter  is  inertia.  If  the  corpuscles  possess  inertia  equal  to 
one-thousandth  of  the  mass  of  the  hydrogen  atom,  it  follows 
that  an  assembleage  of  a  thousand  corpuscles  might  represent 
in  inertia  and  other  properties  the  hydrogen  atom  as  we 
know  it.  This  point  of  view  would  seem  to  account  for  the 
behaviour  of  matter  towards  X-rays,  cathode  rays,  and  the 
rays  from  radio-active  substances.  For  if  the  corpuscle  rather 
than  the  chemical  atom  were  incidental  in  causing  the  absorp- 
tion, the  latter  would  depend  on  the  number  of  the  corpuscles 
in  the  path  of  the  ray,  and  would  be  independent  of  the  nature 
of  the  complexes — chemical  atoms  or  molecules — into  which 
these  corpuscles  were  grouped.  This  view  of  the  constitution 
of  matter  is,  of  course,  far  from  established,  and  mainly 
doubtful  points  require  further  elucidation.  These  cannot  be 
here  considered,  but  the  nature  of  positive  as  opposed  to  nega- 
tive electrification  may  be  mentioned  as  one  of  the  most 
fundamental.  The  above  is  to  be  regarded  as  merely  a 
superficial  summary  of  some  of  the  more  important  physical 
aspects  of  the  work  on  the  conduction  of  electricity  through 
gases,  and  the  nature  of  electrons  or  corpuscles.  The 
mathematical  analysis  upon  which  much  of  the  reasoning 
depends  is  beyond  the  scope  of  the  present  work,  and  the 
original  authorities  must  be  consulted. 

It  may  at  once  be  pointed  out  that  the  theory  of  atomic 
disintegration,  to  which,  in  the  succeeding  chapters,  the  study 
of  radio-activity  will  lead,  is  independent  of  the  electrical  or 
electronic  view  of  atomic  constitution.  It  postulates  no  view 
of  atomic  structure  beyond  the  original  conception  of  Dalton; 
greatly  limits  the  field  of  speculation  on  the  one  hand, 
and,  on  the  other,  it  raises  new  problems  of  its  own 
which  any  satisfactory  theory  will  have  to  account  for. 
The  term  "disintegration"  is  indeed  little  more  than  a 
convenient  and  short  means  of  expressing  certain  experimental 
facts.  It  is  not  until  we  enquire  as  to  the  ultimate  cause  of 
radio-activity,  and  seek  a  knowledge  of  the  forces  at  work 
which  bring  about  the  observed  disintegration,  that  we  enter 
a  region  to  which  the  term  hypothec  in  the  ordinary  sense  of 
a  probable  explanation  would  apply. 


CHAPTER    IV. 


METHODS  OF  MEASURING   RADIO-ACTIVITY. 

Electrical,  Photographic  and  Fluorescence  Methods  of  Measurement. — 
Different  Fluor escers  behave  Differently  to  the  Three  Types  of  Rays. — 
Electrical  Method  of  Measurement. — Apparatus  of  Rutherford  and  of 
Curie.— The  Use  of  the  Gold-leaf  Eh  ctroscope.— Distinction  between 
lonisation  Currents  and  Leaks  due  to  Defective  Insulation. 

The  means  of  investigating  the  rays  from  radio-active 
substances  are  similar  to  those  employed  for  the  X-rays, 
and  may  be  divided  into  three  classes :  the  electrical,  photo- 
graphic and  fluorescence  methods  respectively.  The  first 
only,  the  electrical  method,  is  suitable  for  accurate  quanti- 
tative investigations  of  radio-activity,  but  the  photographic 
and  fluorescence  methods  have  frequently  been  employed  for 
rough  comparisons  and  for  experiments  in  which  accurate 
measurements  of  the  intensity  of  the  rays  are  not  required. 
Thus  much  of  the  work  on  the  magnetic  and  electric  deviation 
of  the  /2-rays  by  Becquerel  has  been  done  by  causing  the  latter  to 
trace  their  paths  on  a  photographic  plate.  It  must  be  pointed 
out  that  the  relative  strengths  of  the  different  types  of  rays 
are  often  very  different  with  the  three  methods.  Thus  the 
electrical  method  depends  on  the  absorption  of  the  rays 
by  the  air,  the  energy  of  the  rays  going  to  produce  ionisation, 
and  with  this  method  the  easily  absorbed  or  a-rays  give 
by  far  the  greater  effect.  These  same  rays,  on  the  other 
hand,  give  relatively  very  little  effect  on  the  photographic 
plate.  Thus  the  a  rays  of  uranium  do  not  perceptibly 
affect  a  photographic  plate,  although  they  contribute  over 
99  per  cent,  of  the  total  electrical  effect  when  examined 
under  ordinary  circumstances  (Soddy,  Jour.  Chem.  Soc.,  1902, 


58  RADIO-ACTIVITY. 

81,  p.  860).  This  is  probably  due  to  the  fact  that  the  rays 
are  unable  to  penetrate  the  gelatin  film  sufficiently  to  affect 
appreciably  the  sensitive  silver  salt. 

Many  fluorescent  substances  behave  differently  under  the 
action  of  the  a  and  /2-rays.  Thus,  zinc  sulphide  (Sidot's 
hexagonal  blende)  is  extremely  sensitive  to  the  action  of  the 
a-rays  and  much  less  to  the  action  of  the  /?-rays,  while  with 
barium  platinocyanide  and  willemite  (zinc  silicate)  the  contrary 
holds  true.  The  diamond  glows  by  the  action  of  the  a-rays. 

Other  fluorescers  are  kunzite,  a  transparent  gem-like  variety 
of  spodumene  (lithium  aluminium  silicate)  which  fluoresces  a 
rose  yellow  ;  sparteite,  a  form  of  calcite  containing  manganese, 
which  shows  a  remarkable  deep  orange-red  fluorescence  ;  and 
scheelite,  a  native  calcium  tungstate,  which  gives  a  fine  deep 
blue  colour.  The  first  two  hardly  seem  to  respond  to  the 
a-rays  of  radium,  being  excited  solely  by  the  /2-rays.  The 
platinocyanides  furnish  at  once  the  most  beautiful  and  intense 
fluorescent  effects  with  the  ft  and  y-rays  of  radium.  Being- 
transparent,  the  whole  body  of  the  matter  adds  to  the  light 
produced  by  a  penetrating  type  of  radiation.  But  it  is 
rather  a  curious  fact  that  magnesium  platinocyanide,  which  is 
one  of  the  best  fluorescers  under  the  X-rays,  does  not  respond 
to  the  radium  rays.  Of  the  other  platinocyanides,  barium  and 
calcium  fluoresce  green,  sodium  lemon-yellow  and  lithium  red, 
the  same  as  for  the  X-rays.  If  the  salts  are  in  the  form  of 
large  crystals  most  beautiful  effects  can  be  obtained,  even  with 
only  a  few  milligrammes  of  the  pure  radium  compounds. 

The  electrical  methods  alone  demand  a  more  detailed  con- 
sideration. It  has  been  shown  in  previous  articles  that,  to 
obtain  definite  results,  the  conditions  must  be  such  that 
the  "  saturation  current"  is  measured,  as  this  current  is 
an  expression  of  the  number  of  ions  produced  per  second, 
and  the  latter  is  proportional  to  the  intensity  of  the  radia- 
tions absorbed  by  the  air.  Rutherford,  in  the  research 
in  which  he  showed  that  the  ionisation  from  uranium 
was  of  the  same  character  as  that  produced  by  X-rays 
(Phil.  Mag.,  1899,  V.,  47,  p.  109),  showed  also  that  the 
ionisation  was  proportional  to  the  pressure  of  the  gas,  while 
the  absorption  of  the  rays  was  also  proportional  to  the  pressure. 
For  different  gases,  although  for  thin  layers  the  ionisation 


METHODS   OF  MEASVE1KO  RADIO-ACTIVITY.         59 


increases  as  the  density  of  the  gas,  for  thicker  layers  the  total 
ionisation  when  all  the  rays  are  absorbed  reaches  a  maximum, 
and  then  is  approximately  the  same  for  all  gases.  A  very 
careful  series  of  measurements  by  Strutt  (Phil.  Trans.,  A.  196, 
p.  507,  1901)  has  led  to  the  conclusion  that,  with  the  exception 
of  hydrogen,  all  gases  are  ionised  by  the  various  rays  from 
radio-active  substances  to  an  extent  sensibly  proportional  to 
the  density  and  independent  of  the  nature  of  the  gas.  With 
hydrogen  the  ionisation  is  greater  than  it  should  be  if  propor- 
tional to  the  density,  and  this  divergence  is  greater  for  the 
a  than  for  the  /3-rays.  We  may  conclude,  as  a  first  approxi- 
mation, that  the  energy  required  to  produce  an  ion  is  the  same 
for  the  ions  of  all  gases  except  hydrogen. 


Earth 


Earth 


Earth 


FIG.  11. 


Fig.  11  represents  an  apparatus  constantly  employed  by 
Rutherford  for  the  electrical  measurement  of  radio-activity. 
The  two  plates  are  each  of  about  36  sq.  cm.  area,  and  a 
known  weight  of  the  radio-active  substance  in  fine  powder  is 
sifted  uniformly  over  the  lower  plate,  which  is  insulated  and 
connected  with  one  pole  of  a  battery  of  300  volts,  the  other 
pole  being  earthed.  The  upper  plate  is  about  5cm.  from  the 
lower,  and  is  connected  with  one  pair  of  quadrants  of  a  Kelvin 
electrometer  of  the  White  pattern,  the  other  pair  being  con- 
nected to  earth.  This  upper  plate  is  most  carefully  insulated, 
and  the  whole  apparatus  is  placed  in  a  metal  box,  provided 


60 


RADIO- A  GT1 VIT  Y. 


with  a  door  at  the  side  through  which  the  lower  plate  can  be 
inserted  or  removed.  The  saturation  current  through  the  air, 
between  the  upper  and  lower  plates,  furnishes  a  measure  of 
the  radio-activity.  To  use  the  apparatus,  both  pairs  of  quad- 
rants are  at  first  connected  to  earth ;  the  earth  connection  of 
the  pair  connected  to  the  upper  plate  is  then  broken,  and  the 
rate  of  deflection  of  the  electrometer  needle  from  zero  is 
measured  by  means  of  a  stop-watch  by  noting  the  time  taken 
for  it  to  pass  over  a  given  number  of  divisions  of  the  scale. 
The  deflection  of  the  electrometer  needle  is  a  measure  of  the 
difference  of  potential  between  the  two  pairs  of  quadrants, 
and,  since  the  capacity  of  the  system  is  constant,  of  the  charge 


Earth 


Earth 


FIG.  12. 


communicated  to  the  one  pair  when  the  other  is  kept  con- 
nected to  earth.  The  rate  at  which  this  charge  is  communi- 
cated is  a  measure  of  the  current  through  the  gas.  0-5  gramme 
of  uranium  or  thorium  oxide  in  this  apparatus  produces  a 
current  of  approximately  10~u  amperes.  The  measurements 
are  usually  comparative,  the  radio-activity  of  some  standard, 
such  as  a  known  weight  of  uranium  oxide  spread  over  a 
given  area,  being  compared  with  the  activity  of  the  substance 
to  be  measured.  In  this  way  the  sensitiveness  of  the  electro- 
meter, which  varies  from  day  to  day  according  to  the  extent 
to  which  the  needle  is  charged,  is  eliminated.  M.  and  Mme. 
Curie  use  a  similar  apparatus,  which  is  represented  in  Fig.  12, 
in  which  the  charge  communicated  to  the  one  pair  of  quad- 
rants by  the  current  through  the  air  under  the  action  of  the 


METHODS   OF  MEASURING  RADIO-ACTIVITY.         61 

radiations  being  measured,  is  balanced  by  the  communication 
of  a  similar  charge  of  opposite  sign,  so  that  the  electrometer 
needle  shows  no  deflection.  This  is  effected  by  means  of  a  quartz 
electric  balance  which  consists  of  a  quartz  lamina,  which  can  be 
subjected  to  a  known  tension  by  means  of  weights,  the  two  sides 
of  the  lamina  receiving  opposite  charges  of  electricity,  the 
amount  of  which  is  known  for  any  known  weight.  The  appli- 
cation of  the  weight  is  so  regulated  by  the  hand  that  at  each 
instant  the  charges  of  opposite  sign  communicated  to  the  electro- 
meter are  equal,  and  the  time  from  the  commencement  to  the 
completion  of  the  application  of  the  tension  furnishes  a  measure 
in  absolute  units  of  the  intensity  of  the  current  flowing  through 
the  air  under  the  action  of  the  radio-active  substance.  In  this 
way  the  operation  is  performed  by  a  null  method,  and  is  inde- 
pendent of  the  sensitiveness  of  the  electrometer. 

For  delicate  work  an  electrometer  designed  by  Dolezalek 
and  constructed  by  Herr  Bartels  of  Gottingen  has  been  much 
used.  It  is  very  small,  and  the  needle  is  of  silvered  paper 
suspended  by  a  quartz  thread.  If  the  latter  is  made  very  fine 
the  sensitiveness  of  the  instrument  is  very  high,  and  may  be  as 
much  as  20,000  divisions  of  the  scale  for  one  volt  P.D.  between 
the  quadrants.  Great  precautions  against  external  disturb- 
ances due  to  accidental  electrification  of  the  surroundings  must 
be  taken  in  working  with  these  very  sensitive  instruments. 

The  simplest  means  of  measuring  radio-activity  electrically 
is  by  means  of  a  gold-leaf  electroscope,  and  this  instrument  is 
very  useful  in  certain  cases,  although  its  application  is  some- 
what limited.  The  results  obtained  with  it  need  very  careful 
scrutiny  if  accidental  errors  are  to  be  avoided.  It  can,  however, 
be  arranged  to  detect  far  feebler  ionisation  than  the  most  sensi- 
tive electrometer,  with  the  exception,  perhaps,  of  the  Dolezalek, 
and  has  proved  invaluable  in  many  important  researches. 
Fig.  13  represents  an  electroscope  suitable  for  the  testing  of 
radio-active  substances.  It  is  modelled  after  one  first  employed 
by  C.  T.  R.  Wilson.  It  consists  of  a  tin  can  with  a  removable 
lid,  E,  at  the  bottom  for  the  insertion  of  the  substance  to  be 
tested.  A  paraffined  rubber  cork,  H,  is  pierced  in  the  centre  by 
the  metal  wire  G,  which  carries  at  its  end  a  rod  of  fused  quartz, 
A.  A  thin  brass  strip,  B,  to  which  a  single  gold-leaf,  C,  is 
attached,  is  fastened  to  the  lower  end  of  the  quartz  rod.  The 


62 


RADIO-ACTIVITY. 


charge  is  communicated  through  the  rod  D,  which  can  be 
turned  by  the  ebonite  handle  F  so  as  to  make  contact  with  B. 
When  the  electroscope  is  charged  this  connection  is  broken, 
and  the  rods  C  and  G  and  the  outside  of  the  metal  vessel  are 
connected  to  earth.  The  electroscope  then  represents  a 
perfectly  isolated  charged  system,  completely  surrounded  by 
metal  and  free  from  all  external  disturbances.  Unless  the 
electroscope  is  required  to  be  made  airtight,  it  is  convenient  to 
insert  a  short  paraffined  glass  tube  into  the  cork  to  serve 
as  a  bearing  in  which  the  chargirig-rod  turns.  If  the 
latter  is  pierced  directly  through  the  cork  the  rod  is  apt 
to  stick  annoyingly  to  the  rubber.  The  rate  of  collapse 


FIG.  13. 

of  the  gold-leaf  is  observed  by  means  of  a  reading  micro- 
scope provided  with  a  micrometer  scale,  through  windows 
in  the  case  of  the  instrument.  For  the  measurement  of  radio- 
activity the  substances  to  be  compared  can  either  be  placed 
inside  the  electroscope  by  spreading  known  weights  over  suit- 
able metal  discs  of  similar  area,  or,  in  the  case  of  more  active 
preparations,  can  be  placed  at  a  fixed  distance  outside  the 
electroscope  so  that  the  rays  have  to  penetrate  the  metal  walls. 
If  the  deflections  are  only  taken  between  certain  limits,  it 
is  found  that  they  are  very  approximately  proportional  to  the 
potential  of  the  leaves.  If  the  dimensions  of  the  instrument 
are  not  too  large  and  the  radio-activity  is  not  too  great,  the 


METHODS  OF  MEASURING  RADIO-ACTIVITY.         63 

saturation  current  through  the  gas  is  attained  and  the  rate  of 
collapse  is  uniform  between  these  limits.  But  all  of  these 
considerations  must  be  continually  borne  in  mind  in  working 
with  an  electroscope  :  The  potential  required  to  make  the 
leaf  diverge  to  a  convenient  extent  is  usually  about  200  to 
400  volts.  For  accurate  work  the  uniform  rate  of  collapse  of 
the  leaf  over  the  part  of  the  scale  employed  must  be  frequently 
verified  with  some  standard  quantity  of  uranium  oxide.  A  far 
more  serious  source  of  error  is  the  possibility  of  the  insulation 
of  the  leaves  deteriorating  during  a  series  of  measurements, 
and  so  simulating  the  effect  being  looked  for.  For  this  reason 
the  natural  leak  of  the  instrument  when  no  active  substance  is 
present  must  be  frequently  redetermined.  In  working  with 
an  electrometer  an  ionisation  leak  is  at  once  distinguished  from 
a  conduction  leak,  due  to  defective  insulation,  by  increasing  the 
voltage.  The  former,  being  the  saturation  current,  is  not  appre- 
ciably increased,  whereas  the  latter  is  proportional  to  the  voltage. 

The  a-rays  are  so  easily  absorbed  that  the  radiation  does 
not  much  increase  beyond  a  certain  thickness  of  the  layer  of 
the  active  substance.  To  make  the  measurements  of  different 
substances  as  comparative  as  possible,  the  smallest  possible 
weight  spread  over  a  large  area  should  be  employed.  But 
even  then  the  activities  of  different  compounds  of  different 
densities  and  states  of  division  can  only  be  approximately 
compared.  On  the  other  hand,  the  change  of  the  activity  of 
any  given  preparation  with  time  can  be  very  accurately  studied 
by  leaving  it  undisturbed  on  its  original  plate  throughout  the 
course  of  the  measurements. 

For  the  measurement  of  more  active  preparations  than 
uranium  and  thorium  the  sensitiveness  of  the  measuring 
instruments  must  be  decreased  by  connecting  them  in  parallel 
with  suitable  capacities.  But  it  must  be  remembered  that  the 
voltage  required  to  produce  the  saturation  current  increases  as 
the  current  increases.  For  work  with  preparations  of  uranium 
a  greater  voltage  than  300  is  seldom  required,  and  this  can  be 
obtained  by  means  of  a  battery  of  Plante  cells  charged  in 
parallel  and  discharged  in  series.  But  for  work  with  radium 
much  higher  voltages  are  necessary  to  produce  the  saturation 
current,  unless  the  intensity  of  the  rays  is  cut  down  by 
absorption  in  suitable  screens. 


CHAPTER    V. 


THE  a,  (3  AND  y-RAYS. 

General  Consideration  of  the  a,  £  and  y-Rays. — y-Rays. — Relation  between 
y  and  fi-Rays. — Explanation  of  High  Penetrating  Power  of  y-Ray  on 
the  view  that  it  is  an  X-Ray  Pulse. — fi-Rays — Photographic  Action — 
Magnetic  and  Electrostatic  Deviation — Value  of  e/m  and  v. — Charge 
carried  by  the  fi-Ray. — Self-electrification  of  Radium. — Kaufmann's 
Work  on  (3-Rays  Approaching  the  Velocity  of  Light. — a-Rays. — 
Curves  of  Penetrating  Powers  of  Various  Types  of  a-Rays. — Diminu- 
tion of  Penetrating  Power  with  Distance  Traversed. — Magnetic  and 
Electrostatic  Deviation. —  Value  of  e/m  and  v. — Charge  carried  by 
a-Rays. — The  Spinthariscope.— Confirmation  of  the  Corpuscular 
Theory  of  Electricity. 

It  has  been  stated  that  the  rays  from  the  radio-active  sub- 
stances, with  the  exception  of  polonium,  are  complex,  and 
three  types,  the  a,  ft  and  y-rays,  have  been  distinguished.  The 
earliest  work  on  this  subject  was  done  by  Rutherford  (Phil. 
Mag ,  1899,  V.,  47,  p.  109),  who  showed  the  complexity  of  the 
rays  from  uranium  by  measuring  the  ionisation  from  a  bare 
layer  of  the  compound,  and  comparing  it  with  that  produced 
when  the  layer  is  covered  with  successive  layers  of  metal  foil 
of  uniform  thickness.  The  apparatus  employed  was  that 
described  in  the  last  chapter  (Fig.  11).  Fig.  14  represents  the 
curve  obtained  by  plotting  the  intensity  of  the  rays  trans- 
mitted on  the  vertical,  and  the  number  of  layers  of  aluminium 
foil  of  thickness  0 '000 12cm.  on  the  horizontal  axis.  It  will  be 
seen  that  the  radiation  is  quickly  diminished  to  a  few  per 
cent,  of  the  original  value  and  after  15  layers  of  foil  have  . 
been  traversed  remains  constant  at  a  minimum  value.  This 
shows  that  the  radiation  is  complex  and  consists  of  an 
easily-absorbed  kind,  which  was  called  the  a  radiation,  and 


66 


RADIO- ACTIVITY. 


a  much  more  penetrating  kind,  which  was  called  the  £  radia- 
tion. The  easily-absorbed  or  a-rays  contribute  over  95  per 
cent,  of  the  total  ionisation,  and  are  completely  stopped 
by  15  to  20  layers  of  the  foil.  The  more  penetrating  or 
fi  radiation  is  able  to  pass  through  50  layers  with  hardly 
any  absorption.  The  latter  was  found  to  be  rather  more  than 
100  times  more  penetrating  in  character  than  the  former,  the 
a-rays  being  reduced  to  half  value  by  passage  through 
aluminium  foil  O'OOOScm.  thick,  the  /?-rays  by  passage  through 
0-05cm.  of  aluminium.  In  the  case  of  the  thorium  radiation 


40 


35 


30 


20 


15 


10 


Uranium 


Oxide 


5         10       15        20        25        30       35        40        45        50 
Thickness  of  Aluminium:  each  division  O-OOOlZcm. 

FIG.  14. 


the  a-rays  are  able  to  pass  through  -0005cm.  of  aluminium 
before  being  reduced  to  half-value.  The  /?-rays  from  thorium 
constitute  a  far  smaller  proportion  of  the  total  radiation  than  in 
the  case  of  uranium.  The  ratio  of  the  ft  to  the  a-rays  for 
the  latter  is  four  times  greater  than  for  thorium  (compare 
.Rutherford  and  Grier,  Phil  Mag.,  1902,  VI.,  4,  p.  326).  The 
existence  of  these  two  types  of  rays  has  since  been  observed 
for  radium,  and,  in  addition,  a  more  penetrating  type  than 
either,  known  as  the  y-rays  was  first  noticed  by  Villard  for 


THE  a,  ft  AND  y-RAYS.  67 

this  element  (Comptes  liendus,  1900,  130,  p.  1,010).  y-rays 
have  also  been  detected  by  Rutherford  in  the  radiations  of 
thorium  and  uranium,  but  large  quantities  of  these  substances 
must  be  used  and  the  finest  and  most  sensitive  electrical 
methods  employed  in  order  to  detect  them.  The  y-rays  are, 
roughly  speaking,  about  100  times  more  penetrating  than  the 
/3-rays,  being  cut  down  to  half  value  by  6cm.  or  7cm.  of  glass 
or  aluminium. 

In  addition  to  their  varying  powers  of  penetrating  matter, 
the  three  types  of  rays  are  characterised  by  different  behaviour 
under  the  influence  of  the  magnetic  and  of  the  electrostatic  field. 
The  /?-rays  are  easily  deflected  into  circular  paths,  and  in  this 
respect,  and  in  the  direction  of  the  deflection,  they  resemble 


ic 


FIG.  15. 

the  cathode  rays.  The  a-rays,  at  first  thought  to  be  un- 
deflected,  have  now  been  shown  by  Rutherford  (Phil.  May., 
VI.,  5,  p.  177,  1903)  to  be  slightly  deviated  by  intense 
magnetic  and  electrostatic  fields,  and  the  deviation  is  in 
the  opposite  sense  to  that  of  the  3  or  cathode  ray.  The 
y-rays  remain  quite  undeflected.  These  facts  have  been  con- 
veniently represented  by  Mme.  Curie  by  a  diagram  (Fig.  15)  in 
which  the  rays  are  represented  to  be  issuing  from  radium 
placed  in  a  deep  cavity  in  a  block  of  lead  under  the  action  of 
an  intense  magnetic  field  at  right  angles  to  the  direction  of  the 
beam  and  the  plane  of  the  paper.  The  thin  pencil  of  unde- 
flected rays  represents  the  relatively  feeble  y-rays.  The  intense 
pencil  slightly  deviated  the  a-rays,  and  the  circular  paths  of 

F2 


68  JiADIO- ACTIVITY. 

varying  radii  the  /3-rays,  deflected  in  the  opposite  direction  to- 
the  a.  It  must  be  noted  that  the  scale  of  the  deviation  of  the 
a-rays,  compared  with  that  of  the  ft  is  enormously  exaggerated 
in  the  diagram  for  the  sake  of  clearness. 

y-Eays. — It  will  be  convenient  to  consider  the  three  types 
of  rays  in  detail  in  the  inverse  order  of  their  importance,  and 
to  commence  with  the  least  known  and,  except  in  the  case  of 
powerful  radium  compounds,  least  obvious  type — the  y-rays. 
They  are  characterised  by  extraordinarily  great  penetrating 
power,  being  able  to  pass  through  7cm.  of  lead,  19cm.  of 
iron  and  150cm.  of  water  before  they  are  reduced  in  inten- 
sity to  1  per  cent,  of  the  original  (Rutherford,  Nature,  1902, 
66,  p.  318).  No  deviation  of  these  rays  has  been  detected  in 
the  most  powerful  magnetic  fields  it  is  possible  to  obtain,  and 
in  this  respect  they  are  more  nearly  allied  to  the  X-rays  than 
to  the  a  and  /?-rays  ;  but  in  their  absorption  by  different  kinds 
of  matter  they  exhibit  an  almost  complete  parallelism  to  the 
5-rays.  The  simple  law  enunciated  in  the  first  chapter — and 
which  for  shortness  will  be  called  the  Density  Law — that  the 
absorption  of  the  new  types  of  radiation  by  matter  is  pro- 
portional to  its  density,  and  independent  of  its  nature,  is 
followed  strictly  by  none  of  the  known  types.  The  exceptions 
are,  however,  different  for  the  different  types.  Thus,  for  all 
gases,  with  the  exception  of  hydrogen,  the  absorption  of  the 
rays  in  the  gas,  as  measured  by  the  amount  of  ionisation. 
produced,  is  in  good  agreement  with  the  density  law  for  all 
three  types  of  rays.  In  the  case  of  hydrogen,  the  ionisation 
is  about  twice  as  great  for  the  ft  and  y-rays  and  about  three 
times  as  great  for  the  a  rays  as  the  density  would  lead  us  to- 
expect.  But  for  ordinary  X-iays  the  ionisation  in  the  case  of 
heavy  gases  and  vapours — as,  for  example,  chloroform,  methyl 
iodide  and  carbon  tetrachloride— is  from  7  to  15  times  greater 
than  the  density  law  requires  (compare  Strutt,  Proc.  Roy. 
Soc.,  1903,  72,  p.  208). 

At  first  sight  this  seems  to  point  to  the  y-rays  being  more 
nearly  allied  to  the  /2-rays  than  to  the  X-rays.  But  it  must 
be  remembered  that  the  /i-rays  themselves  differ  from  the 
ordinary  cathode  rays  of  the  Crookes  tube  in  possessing  a 
far  higher  velocity,  and  it  is  to  be  expected  that  the  X-ray 
from  radium  produced  from  the  /3-ray  at  the  moment  of  its 


THE  a,  p  AND  y-KA  YS.  69 

sudden  expulsion  will  also  differ  from  the  X-rays  of  the 
Crookes  tube.  It  has  recently  been  found  (A.  S.  Eve,  Nature, 
March  10,  1904)  that  if  the  absorption  of  only  the  most 
penetrating  X-rays  produced  from  a  "  hard  "  tube  in  dense 
gases  is  measured,  the  deviations  from  the  density  law 
noticed  for  the  easily-absorbed  X  rays  to  a  large  extent 
disappear.  That  is,  very  highly  penetrating  X  rays  are 
similar  to  the  y-rays  of  radium  in  their  absorption  by 
dense  gases.  Rutherford  has  pointed  out  (loc.  cit.)  that  it 
is  to  be  expected  that  a  narrow  electromagnetic  pulse — 
i.e.,  a  penetrating  type  of  X-ray  — will  be  generated  at  the 
sudden  expulsion,  with  extreme  velocity,  of  the  ft-  corpuscle 
from  radium.  On  the  other  hand,  on  account  of  the  great 
penetrating  power  of  the  /3-corpuscle,  it  is  impossible  to 
stop  it  suddenly,  even  by  the  densest  matter,  and  the  X-rays 
produced  by  it  on  impact  with  matter  will  be  broad  pulses 
of  non-penetrating  character,  which  experimentally  would  be 
difficult  to  detect.  There  is  thus  very  strong  evidence  for 
supposing  that  the  y-rays  of  radium  are  to  the  /2-rays  as 
the  X-rays  of  a  Crookes  tube  are  to  the  cathode  rays,  with 
the  difference  that  the  y-rays  accompany  the  expulsion  of  the 
/?-ray,  whereas  the  X-rays  accompany  the  stoppage  of  the 
cathode  rays.  This  point  of  view  is  borne  out  by  the  fact 
observed  by  Rutherford  that  the  y-rays  of  radium  are,  under 
all  circumstances,  proportional  to  the  /2-rays.  There  is  always 
a  complete  parallelism  in  the  intensity  of  the  ft  and  y-rays ; 
and  throughout  the  various  processes  of  decay  and  recovery, 
which  will  be  considered  later,  the  ratio  between  these  types 
remains  unchanged. 

It  may  be  pointed  out  here  that  these  new  facts  strongly 
confirm  the  view  that  will  be  developed  later  as  to  the  nature 
of  radio-activity.  X-rays  should  result  from  cathode  rays 
whenever  the  corpuscles  constituting  the  latter  are  accelerated 
either  positively  or  negatively,  and  are  the  more  penetrating 
the  greater  the  acceleration.  Sir  George  Stokes  pointed  out 
that  X-rays  should  accompany  the  production  of  the  cathode 
rays  as  well  as  their  stoppage.  But  in  the  Crookes  tube  the 
cathode  ray  corpuscle  acquires  its  velocity  gradually  as  it  moves 
under  the  influence  of  the  electric  field.  Its  acceleration  is, 
therefore,  relatively  small,  and  the  pulse  which  results  has 


70  RADIO-ACTIVITY. 

feeble  penetrating  power  and  is  incapable  of  detection.  When 
the  corpuscle  strikes  the  anti-cathode  the  resulting  acceleration 
is  very  great,  and  a  pulse  of  high  penetrating  power  is  emitted, 
giving  rise  to  the  penetrating  X-rays  which  escape  from  the 
tube.  In  the  radio-active  substances  the  converse  holds  true. 
The  /5-corpuscle  is  suddenly  expelled  as  the  result  of  the 
explosion  of  the  atom,  and  the  value  of  the  acceleration  is  pro- 
bably far  higher  than  that  of  the  cathode  ray  corpuscles  at  the 
anti-cathode.  In  consequence,  the  y-rays  are  far  more  pene- 
trating than  the  X-rays.  But  the  acceleration  suffered  by  the 
/3  corpuscles  when  they  encounter  obstacles  is  relatively  small 
because  they  are  very  penetrating  and  difficult  to  stop.  Hence, 
the  X-rays  resulting  from  the  stoppage  of  the  /?-rays  will  be  of 
low  penetrating  power,  and  will  not  be  capable  of  detection, 
for  the  y-rays  can  only  be  detected  by  virtue  of  their  extreme 
penetrating  power  and  of  the  possibility  of  sorting  them 
out  from  all  the  other  types.  Their  effects  are  insignificant 
compared  with  that  of  the  other  two  types. 

p-mys. — On  account  of  their  relatively  intense  photographic 
action,  and  of  their  power  of  penetrating  opaque  screens  of 
considerable  thickness,  the  /3  radiation  was  at  first  the  most 
studied.  In  much  of  the  earlier  photographic  work,  the 
term  radiation  must  be  interpreted  to  mean  /3  radiation, 
and  the  work  considered  to  refer  only  to  this  type.  The 
existence  of  rays  easily  deviable  in  the  magnetic  field  in 
the  radiations  from  radium  was  discovered  simultaneously  by 
Giesel,  Meyer  and  von  Schweidler,  and  Becquerel.  The  latter 
subsequently  (Comptes  Eendus,  1900,  130,  p.  1,584)  showed  the 
same  to  be  true  of  the  radiations  from  uranium.  Rutherford 
and  Grier  (Phil  Mag,,  1902,  VI.,  4,  p.  315)  proved  that  thorium 
also  gave  out  deviable  rays,  although  the  proportion  of  /3-rays 
in  the  radiations  of  the  latter  is  much  less  than  in  that  of 
uranium.  This  explains  the  fact  that,  although  the  radio- 
activities of  thorium  and  uranium  are  very  similar  when  tested 
by  the  electrical  method,  the  former  is  much  less  active  than 
the  latter  to  a  photographic  plate.  In  all  cases  it  is  the  photo- 
graphically active,  penetrating  /2-ray  which  is  deviated  by  a 
magnetic  field.  Curie  (Comptes  fiendus,  1900,  130,  p.  73)  using 
the  apparatus  shown  in  Fig.  1 6  showed  that  the  easily-absorbed 
type  of  radiation  from  radium  is  not  appreciably  affected  by 


THE  a,  p  AND  y-RAYS. 


71 


a  magnetic  field,  whereas  the  penetrating  radiation  was  com- 
pletely deviated.  A  is  the  radium  preparation,  BB'  are  blocks 
of  lead  of  varying  heights,  PP'  plates  connected  to  the  electro- 
meter and  battery  respectively  between  which  the  ionisation 
due  to  the  rays  is  measured.  A  powerful  electromagnet 
(not  shown  in  the  figure)  is  arranged  so  as  to  deviate  the 
/3-rays  into  the  lead  blocks  BB'.  One  pole  of  the  magnet 
would  be  placed  beneath  and  the  other  above  the  plane  of  the 
paper  to  secure  this  effect.  If  the  distance  AD  is  greater 
than  7cm.,  all  the  rays  are  deviated  by  a  magnetic  field,  and 
are  absorbed  by  the  lead  blocks.  For  smaller  distances  only 


Battery 
f- 


B 


E  E 

FIG.  16. 


a  fraction  is  deviated.  This  proves  that  the  non-deviable  or 
a  radiation  of  radium,  is  completely  absorbed  in  7cm.  of  air. 
Under  the  conditions  of  experiment  the  effect  of  the  y-rays 
was  too  small  to  be  detected.  Becquerel  investigated  the 
magnetic  deviation  of  the  /3-ray  of  radium  by  placing  the 
substance  in  a  metal  capsule  upon  the  sensitive  film  of  a 
photographic  plate  wrapped  in  paper  and  placed  in  an 
exhausted  space.  Under  these  circumstances  the  rays  formed 
a  pencil  proceeding  normally  away  from  the  plate,  and  by 
applying  a  magnetic  field  they  could  be  curved  into  circles 
cutting  the  plate  at  some  distance  from  the  point  of  origin. 
The  disposition  of  the  experiment  is  the  same  as  in  the 


72  RADIO- A  CT1 VITY. 

diagram,  Fig.  15,  where  AC  is  the  photographic  plate.  Fig.  17 
represents  a  negative  so  obtained  by  first  applying  the  field  in 
the  one  direction  and  then  in  the  other.  The  central  black 
spot  represents  the  position  of  the  radio-active  substance  ; 
the  curved  areas  of  darkening  on  either  side  the  points  where 
the  rays,  travelling  upwards  initially,  are  curved  round  by  the 
field  and  cut  the  plane  of  the  paper.  Becquerel  (Complex 
Rendus,  1900,  130,  p.  809)  showed  that  the  /3  radiation  of 
radium  is  complex,  and  different  rays  suffer  different  deviations 
in  a  uniform  magnetic  field.  From  the  equation  ~H.p  =  mv/e 
(compare  p.  50),  Becquerel  found  values  for  mvje  varying 
from  350  (for  rays  capable  of  penetrating  O'Olmm.  aluminium) 
to  2,600  (for  rays  penetrating  0-1 3mm.  of  lead).  Becquerel 
identified  the  /3-rays  with  the  cathode  rays  by  proving  that  they 
were  deviated  in  the  same  sense  as  the  latter  by  an  electrostatic 
field.  He  deduced  v=  1'6  x  1010cm.  per  sec.  for  the  average 


FIG.  17. 

velocity  of  the  rays,  and  this  is  several  times  greater  than  the 
value  found  by  Prof.  Thomson  for  the  velocity  of  the  cathode 
ray.  This  fact  explains  the  much  greater  penetrating  power 
of  the  /3-rays.  The  great  variation  in  penetrating  power  of 
the  /^-rays  of  radium  is  due  simply  to  the  wide  range  of 
velocities  possessed  by  different  rays.  The  value  of  efm  was 
found  to  be  the  same  as  for  the  cathode  ray — viz.,  107, 
and  the  /2-rays  must,  therefore,  be  regarded  as  negatively 
charged  particles  of  mass  one-thousandth  of  the  hydrogen 
atom  moving  at  speeds  comparable  with  that  of  light. 
This  conclusion  has  been  confirmed  by  the  direct  deter- 
mination of  the  negative  charge  carried  by  the  ft  ray  by 
M.  and  Mme.  Curie  (Comptes  Bendus,  1900,  130,  p.  647), 
using  the  apparatus  shown  in  Fig.  18.  The  plate  MM  on 
which  the  /?-rays  from  the  radium  R  impinge  is  completely 


THE  a,  ft  AND  y-RAYS.  73 

enclosed  in  paraffin  and  connected  with  an  electrometer. 
In  these  circumstances  any  charge  communicated  to  the 
plate  will  be  retained,  whereas  if  the  plate  were  exposed 
hare  to  the  strongly  ionised  air  in  the  neighbourhood  of 
radium  it  would  tend  to  be  discharged  as  fast  as  the 
charge  carried  by  the  rays  were  communicated  to  it.  It  is 
found  that  a  negative  charge  is  conveyed  to  the  plate  MM 
from  the  radium  R  through  the  screen  of  thin  metal,  pp,  connected 
to  earth.  The  charge  is  extremely  feeble,  and  can  only  be 
determined  for  very  active  radium  preparations.  It  is  to  be 
noted  that  it  is,  probably,  only  about  the  y0  0*000^  °^  *ne 
charge  carried  by  the  ions  produced  by  the  complete  absorption 
of  the  rays  in  air.  This  fact  cannot  be  too  strongly  empha- 
sised. The  ions  shot  out  by  radium  are  of  the  second  class, 
and  are  detected  by  their  kinetic  energy.  The  part  played  by 
the  air  or  gas  in  the  measurement  of  the  ionisation  produced 


FIG.  18. 

by  radio-activity  is  a  secondary  feature,  which  serves  as  a 
convenient  means  of  measurement.  In  vacuo,  the  ionisation 
produced  by  a  radio-active  substance  would  be  nil,  but  the 
radio-activity  would  be  the  same  as  in  air.  In  hydrogen  the 
ionisation  would  usually  be  feebler  than  in  air  or  carbon- 
dioxide,  but  the  activity  of  the  substance  is  not  affected  by 
the  atmosphere  surrounding  it.  In  the  case  of  the  phenomenon 
of  imparted  activity  already  mentioned  we  measure  the  ions 
produced  by  the  rays  produced  by  the  matter  causing  the 
imparted  radio-activity,  which  is  produced  by  the  emanation 
which  is  produced  by  radium.  The  number  of  the  steps 
involved  in  the  production  of  the  phenomena  observed  is  one 
of  the  initial  difficulties  of  the  subject. 

The  existence  of  the  negative  charge  carried  away  from 
radium  by  the  /3-rays  has  recently  been  shown  as  a  lecture 


74 


RAD  10- A  CTIVIT  Y. 


experiment  by  Strutt  (Phil  Mag.,  1903,  VI.,  6,  p.  588) ,  with 
only  a  small  quantity  of  radium,  by  the  apparatus  represented 
in  Fig.  19.  A  tube,  a,  containing  radium,  is  supported  in  a 
perfectly  exhausted  vessel  by  a  quartz  rod,  b.  To  the  lower 
end  of  the  tube  an  electroscope,  cc,  is  attached,  and  the  surface 
of  the  tube  a  is  made  conducting  with  phosphoric  acid.  The 
positive  charge  left  behind  after  the  expulsion  of  the  /3-rays  is 
communicated  to  the  leaves,  which  diverge  until  they  touch 
the  sides  of  the  vessel  connected  to  earth.  They  then  collapse 
and  again  commence  to  diverge,  this  cycle  of 
operations  being  repeated  indefinitely.  In  an 
apparatus  constructed  by  the  Author,  about 
2  milligrammes  of  pure  radium  bromide  was 
used,  and  the  cycle  of  operations  occupied 
about  four  minutes. 

It  is  to  be  noticed  that  the  charge  thus 
measured  is  an  induced  positive  charge  on 
the  outside  of  the  glass  repelled  to  the  leaves 
by  the  accumulation  of  excess  of  positive 
electricity  within.  The  latter  accumulates 
continuously,  and  so  increases  until  the  strain 
becomes  too  great  and  the  glass  of  the  tube 
is  punctured,  as  in  the  case  of  an  over-charged 
Ley  den  jar.  Several  cases  are  on  record  in 
which  sealed  tubes  of  active  radium  prepara- 
tions have  shattered  with  the  accompaniment 
of  a  bright  spark  and  explosion  after  having 
been  kept  sealed  up  for  some  months. 

Kaufmann  (Naclirichten  der  K.  Gesells. 
der  Wiss.  zu  Gottingen,  1901,  No.  2)  has 
measured  both  v  and  e/m  for  the  more  pene- 
trating kinds  of  /3-rays  emitted  by  radium,  and  obtained  for 
rvalues  between  2-36  and  2'83  x  1010— i  e.,  between  80  and 
95  per  cent,  of  the  velocity  of  light,  Now,  it  has  already  been 
pointed  out  (p.  54)  that  if  the  mass  of  the  corpuscle  is 
electrical  in  origin  and  due  to  the  inertia  of  the  moving  charge, 
it  should,  theoretically,  tend  to  increase  as  the  velocity  of  light 
is  approached,  and  become  infinite  when  that  value  is  attained. 
Now  Kaufmann  found  that  the  ratio  e/m  decreased  from 
1-31  x  107  when  v=  2-36  x  10lctoO'«3  x  107when  t>=2'83  x  1010,. 


FIG.  19. 


THE  a,  /3  AND  y-RA  YS.  75 

and  these  results  are  in  good  agreement  with  the  theory  that 
the  mass  of  the  corpuscle  or  electron — i.e.,  the  negative  atomic 
charge,  which  constitutes  the  cathode  ray  and  the  /?-rays 
from  radio-active  substances — is,  at  least  mainly,  electrical  in 
origin. 

a-rays. — These  are  given  out  by  all  radio-active  sub- 
stances, with  the  possible  exception  of  uranium  X  (p.  85), 
and,  although  the  least  striking,  are  much  the  most  impor- 
tant of  the  three  types,  representing,  in  each  case,  as 
Rutherford  has  shown,  by  far  the  greater  part  of  the  total 
energy  radiated.  In  measurements  by  the  electrical  method, 
unless  special  precautions  are  adopted,  the  effect  of  the  /3 
and  y-rays  is  negligible  compared  with  that  of  the  a-rays. 


1  2  tf  4 

Layers  of  Aluminium  Foil,  0'00036<rw.  thick. 
FIG.  20. 

Throughout  the  book  measurements  of  radio-activity  must  be 
taken  to  mean  measurements  of  the  intensity  of  the  a-rays, 
unless  otherwise  stated.  Their  power  of  penetration  is  always 
very  small,  but  varies  considerably  for  the  different  radio-active 
substances.  The  diagrams  (Figs.  20  and  21)  were  given  by 
Rutherford  and  Miss  Brooks  (Phil.  Mag.,  1902,  VI.  4,  p.  1),  and 
show  the  penetrating  power  of  the  a-rays  of  uranium,  polonium, 
radium,  thorium,  together  with  that  of  the  imparted  radio- 
activities produced  by  the  radium  and  thorium  emanation. 
In  each  case  the  vertical  axis  represents  the  rays  transmitted, 
while  the  horizontal  axis  represents  the  thickness  of  matter 
penetrated,  this  being,  in  Fig.  20,  made  up  of  successive  layers  of 


7(5 


RADIO-ACTIVITY. 


aluminium  foil  of  thickness  000036cm.,  and  in  Fig.  21  of  the 
thickness  of  air  in  millimetres.  It  will  be  seen  that  the  order 
is  the  same  for  the  different  rays  in  the  case  of  aluminium 
and  air,  those  from  uranium  being  the  least  and  from  the 
imparted  activities  the  most  penetrating.  The  density  law, 
that  the  absorption  of  the  rays  by  various  substances  is 
proportional  to  the  density  of  the  latter,  holds  fairly  generally 
for  the  a-rays.  Tin  furnishes  an  exception,  its  opacity  being 
about  the  same  as  for  aluminium,  although  it  is  three  times  as 
dense  as  the  latter.  Tin  is  also  an  exception  in  its  power  of 
absorbing  the  /2-rays,  but  in  the  opposite  direction,  being  three 
times  more  absorbing  than  its  density  would  indicate.  It 
thus  absorbs  /3-rays  nine  times  more  than  the  a-rays  compared 
with  other  metals.  Lead  also  absorbs  the  /3-rays  abnormally, 


100 


0     2     4     6     8    10    12   14    16    18    20    22    24    26   28   30   32   34   26   38   40   42 
Distance  in  Millimetres. 

FIG.  21. 

being  twice  as  opaque  as  other  metals  in  proportion  to  its 
density.  In  this  respect  the  y-rays  resemble  the  /3-rays 
completely. 

A  very  characteristic  property  of  the  a  rays  is  that  they  are 
absorbed  the  more  readily  the  greater  the  thickness  of  matter 
traversed  (compare  Mrne.  Curie,  "  Thesis,"  Chap.  III.  •  Euther- 
ford,  Phil.  Mag.,  1903,  VI.,  5,  p.  114).  The  apparatus  employed 
by  Mme.  Curie  for  the  investigation  of  the  a-rays  of  polonium 
is  shown  in  Fig.  22.  The  object  of  the  experiment  is  to 
measure  the  ionisation  current  due  to  the  rays  between  the 
plates  PP,  P'P'  from  a  polonium  preparation  at  A  when  the 
distance  AT  was  varied.  The  hole  T  in  the  plate  P'P'  is 


THE  a,  /3  AND  y-KA  YS. 


77 


covered  with  the  thinnest  possible  sheet  of  aluminium  foil  (not 
shown)  to  prevent  the  ions  formed  in  CCCC  finding  their  way 
into  the  space  PPP'P'.  When  AT  is  over  4cm.  no  current  passes,, 
indicating  that  the  rays  are  all  absorbed  in  this  thickness  of  air. 
The  appearance  of  the  rays  between  the  plates  is  manifested 
somewhat  suddenly  as  the  distance  AT  is  reduced.  If  suc- 
cessive layers  of  foil  are  placed  over  the  polonium,  the  absorp- 
tion caused  by  the  second  layer  is  greater  than  that  produced 
by  the  first. 

These  considerations  and  the  existence  of  this  peculiarity  for 
a-rays  generally  led  Rutherford  (Phil.  Mag.,  1903,  VI.,  5,  p.  177) 
to  regard  the  a-rays  as  consisting  of  projectiles,  and  to  examine 


them  carefully  in  the  most  intense  magnetic  fields  under  con- 
ditions where  the  slightest  deviation  could  be  detected.  His 
apparatus  is  represented  in  Fig.  23.  The  radium  rays  pass 
upwards  through  a  series  of  very  narrow  slits  placed  between 
the  poles  of  a  very  powerful  electromagnet  and  emerge  inside 
the  gold-leaf  electroscope  placed  above.  A  current  of  hydrogen 
is  kept  flowing  through  the  electroscope  and  the  slits  in  a, 
downward  direction,  and  this  serves  to  prevent  the  diffusion 
of  any  emanation  from  the  radium  into  the  electroscope.*  It 

*  The  aluminium  foil  placed  above  the  top  of  the  slits  serves  the  same 
purpose  as  in  Mme.  Curie's  apparatus  just  described.  It  is  excessively 
thin  and,  being  porous,  offers  no  obstruction  to  the  passage  of  the 
hydrogen. 


78 


RADIO-ACTIVITY. 


.also  greatly  reduces  the  absorption  of  the  a-rays  in  passing 
through  the  slits,  and  so  increases  the  effect  to  be  measured  in 
the  electroscope.  The  effect  due  to  the/?  and  y-rays  is,  on  the 
•Other  hand,  very  much  diminished  by  using  hydrogen,  owing  to 
the  ionisation  in  the  latter  gas  being  much  less  for  a  penetrat- 
ing radiation  than  it  is  in  air.  The  ionisation  due  to  the  a-rays 
is  not  reduced  because  there  is  a  sufficient  thickness  of  gas  to 
•ensure  their  complete  absorption  inside  the  electroscope.  The 
plan  of  the  experiment  was  to  measure  the  rate  of  collapse  of 
ithe  leaves  when  the  radium  salt  was  bare  and  when  covered 


Earth 


from  above. 


FlG.  23. 


with  a  sheet  of  mica  sufficient  to  absorb  the  a-rays,  both  with  and 
without  the  magnetic  field.  Under  proper  conditions  the  rate 
of  collapse  is  the  same  with  the  salt  bare  and  the  magnetic 
field  on  as  with  the  salt  covered  and  the  field  off.  But  in 
the  latter  condition  the  residual  magnetism  was  always 
sufficiently  strong  to  eliminate  the  /?-ray?,  so  that  these, 
under  no  circumstances,  enter  the  electroscope.  In  this  way 
Rutherford  showed  that  the  a-rays  were  completely  deviated 
by  passage  through  slits  4'5cm.  long  and  0'55cm.  wide  placed 


THE  a,  /3  AND  y-£AYS. 


79 


in  a  uniform  magnetic  field  of  8,400  C.G.S.  units.  Deviation 
was  also  obtained  by  an  electrostatic  field;  and  the  direction 
of  deviation  in  each  case  is  opposite  to  that  suffered  by  the 
cathode  ray  or  the  /?-ray  of  radioactive  substances.  The  direc- 
tion of  deviation  was  determined  by  using  the  device  shown 
in  Fig.  24,  in  which  the  openings  of  the  slits  are  half  covered 
from  the  one  side  by  metal  plates,  so  that  a  greater  deflec- 
tion is  necessary  in  the  one  direction  than  in  the  other  if 
no  rays  are  to  enter  the  electroscope.  From  a  combination 
of  his  results  for  both  the  electrostatic  and  electro-magnetic 
deviation,  Rutherford  has  deduced  that  the  velocity  of  the 
charged  particle  v  =  '2'5  x  109cm.  a  second,  while  the  ratio  of 
the  charge  to  the  mass  e/m=6  x  103.  If  e  is  assumed  to 
be  the  "  atomic  charge,"  and  equal  to  the  charge  carried  by 


FIG.  24. 


the  hydrogen  ion  in  electrolysis,  the  mass  of  the  projected 
particle  is  thus  about  1  -6  times  that  of  the  hydrogen  atom.  The 
value  of  H/o  in  these  experiments  was  390,000,  and  this  result 
may  be  expressed  differently  by  the  statement  that  the  a-rays 
would  be  coiled  into  circles  of  39cm.  radius  under  conditions 
\vhich  would  coil  the  cathode  rays  into  circles  of  O'Olcm. 

The  magnetic  deviability  of  the  a-ray  of  radium  has  since 
been  confirmed  by  a  photographic  method  by  Becquerel 
(Comptes  fiendus,  1903,  136,  pp.  199,  431  and  1,517),  and  he  also 
shows  that  the  a-ray  of  polonium  is  deviated  to  a  similar  extent. 
Des  Coudres  also  (Phys.  Zeitschr.,  1903,  p.  483),  working  by  a 
photographic  method  in  vacuo,  has  confirmed  both  the  electro- 
static and  electromagnetic  deviation,  and  obtained  a  value  of 
e/m  in  close  agreement  with  that  given  by  .Rutherford. 


80  RADIO-ACTIVITY. 

Since  the  a-rays  of  all  radio-active  substances  are  very  similar 
in  type,  there  is  strong  evidence  for  considering  them  all  as 
consisting  of  positive-charged  particles  of  mass  about  twice 
that  of  the  hydrogen  atom  travelling  with  a  speed  one-tenth 
of  that  of  light.  Their  large  size  compared  with  that  of  the 
/2-ray  accounts  for  their  small  penetrating  power,  and  their  great 
kinetic  energy  for  their  very  great  ionising  action,  while  their 
projection  character  well  accords  with  the  property  already 
mentioned  of  being  more  easily  stopped  the  greater  the  thick- 
ness of  matter  penetrated. 

This  view  of  the  discrete  nature  of  the  a-radiation  has 
recently  been  confirmed  by  an  experiment  of  Sir  William 
Crookes'  (Cliem.  Neivs,  1903,  87,  p.  ^41),  which  was  discovered 
simultaneously  in  Germany  by  Elster  and  Geitel  (Chem.  Neius, 
1903,  88,  p.  37),  that  the  phosphorescence  of  a  zinc  sulphide 
screen  under  the  action  of  the  a-rays  consists,  when  viewed 
through  a  lens,  of  a  number  of  momentary  flashing  points  of 
light  or  scintillations,  presenting  the  appearance  of  an  inces- 
sant bombardment  of  the  screen  by  a  rain  of  projectiles  from 
the  radium,  each  impact  being  marked  by  a  separate  flash 
of  light.  An  instrument,  known  as  the  "Spinthariscope,"  has 
been  designed  to  show  this  eft'ect. 

We  thus  see  that  the  rays  of  radium  have  furnished  most 
striking  and  unexpected  evidence  of  the  correctness  of  the  views 
adopted  by  Prof.  J.  J.  Thomson,  as  the  result  of  his  investiga- 
tions into  the  relations  between  electricity  and  matter.  The  a-ray 
has  been  shown  to  be  a  positively-charged  particle  of  atomic 
dimensions,  the  /3-ray  a  negatively-charged  particle  of  corpus- 
cular dimensions,  in  accordance  with  the  corpuscular  theory 
of  electricity,  which  requires  that  positive  electrification  does 
not  exist  apart  from  matter,  whereas  negative  electrification  is 
composed  of  discrete  charges  of  far  smaller  mass  than  the 
lightest  atom  of  matter.  In  addition  the  /3-ray  of  radium  has 
afforded  a  welcome  means  of  putting  to  experimental  test  one  of 
the  most  fundamental  consequences  which  follow  from  the  view 
that  an  electric  charge  possesses  inertia  and,  therefore,  mass. 


CHAPTER    VI. 


URANIUM    X   AND   THORIUM   X. 

Radio-activity  an  Atomic  Property. — The  First  Idea  Untenable. — Crookes* 
Uranium  X.— Decay  of  Activity  of  Uranium  X.— Hypothesis  of  Radio- 
active Induction. — Uranium  _ST  gives  only  fi-Rays. — a  Radiation  a 
Specific  Property  of  Uranium. — Thorium  X.—  A  Specific  Type  of 
Matter. — Greater  Part  of  a-Rays,  all  the  (3  Rays,  and  the  Emanating 
Power  due  to  Thorium  X. — Decay  and  Recovery  of  Activity  of 
Thorium  X  and  Thorium. — Continuous  Reproduction  of  Thorium  X. — 
Radio-active  Change.  — Radio-active  Equilibrium. — Radio-active  Con- 
stant \. — Independence  of  Radio-active  Change  towards  all  known 
Agencies. — Curves  of  Decay  and  Recovery  of  Activity  of  Uranium  X 
and  Uranium. — Homogeneity  of  the  Elements  Thorium  and  Uranium. 

It  has  been  shown  in  the  preceding  chapters  that  the 
phenomenon  of  radio-activity  consists  essentially  in  the  incessant 
and  spontaneous  expulsion  of  positively  and  negatively- 
charged  particles  from  the  three  heaviest  chemical  elements 
that  are  recognised — viz.,  radium,  thorium  and  uranium.  The 
velocity  of  both  the  positively  and  negatively-charged  particles 
shot  out  spontaneously  from  the  radio-elements  is  greater 
than  is  ever  impressed  by  external  agencies  upon  the  posi- 
tive and  negative  ions  of  the  electric  discharge,  even  when 
the  latter  are  caused  to  move  in  electric  fields  of  very  great 
intensity.  In  consequence,  the  kinetic  energy  associated  with 
the  a  and  /3  particles  is  greater  than  in  the  case  of  the  ions 
artificially  produced,  and  is,  in  fact,  considered  with  reference 
to  their  mass,  of  a  far  higher  order  of  magnitude  than  is 
associated  with  matter  in  any  other  form. 

We  have  now  to  consider  the  radio-active  matter  from  which 
these  particles  are  projected,  and  from  which  their  energy 
must  in  some  way  be  derived.  It  must  be  first  pointed  out 
that  matter  of  this  kind,  from  which  energy  is  being  con- 
tinuously dissipated  in  a  Dew  and  easily  detected  form,  is 


82  EADIO-ACTIVITY. 

naturally  far  more  easy  to  investigate  and  trace  than  non- 
radio-active  matter.  New  experimental  methods  of  great 
delicacy  are  available  which  are  capable  of  detecting  the 
presence  of  such  matter  in  quantity  so  minute  that  no  other 
indication  of  its  presence  exists.  The  first  fruits  of  the 
researches  in  this  direction  were,  of  course,  Mme.  Curie's  dis- 
coveries of  radium  and  polonium,  which  have  already  been 
discussed.  It  will  be  recalled  that  these  discoveries  resulted 
from  the  view  that  radio-activity  is  an  atomic  property  of  the 
element,  in  that  it  is  exhibited  by  each  radio-element  in  amount 
sensibly  proportional  to  the  quantity  present,  and  independent 
of  its  state  of  chemical  combination,  of  physical  conditions,  and 
of  previous  history.  On  this  view,  a  substance  like  pitch- 
blende, more  radio-active  than  any  known  element,  owes  its 
radio-activity  to  the  presence  of  new  elements  of  proportionally 
great  radio-activity,  and  not  to  any  cause  increasing  the  activity 
of  the  known  radio-elements  contained  therein  beyond  the 
normal  value.  In  spite  of  the  successful  separation  of  these 
predicted  new  elements,  a  series  of  researches,  which  have  now 
to  be  considered,  have  shown  that  the  view  that  radio-activity 
is  an  atomic  property  of  the  element  in  question  cannot  be 
accepted  in  its  original  form.  Mme.  Curie,  in  one  of  her 
earlier  papers,  describes  radio-activity  as  a  property  exhibited 
by  each  atom  of  the  radio-active  element,  which  acts  as  a  con- 
tinual source  of  energy.  ,  This  view  is  now  known  to  be 
untenable,  for  we  shall  see  that  the  normal  radio-activity  of  a 
single  element,  like  thorium,  is  due  to  several  different  types 
of  matter  radiating  simultaneously,  and  the  thorium  contributes 
but  a  small  part  of  the  total  radio-activity. 

The  discrepancy  has  been  reconciled  by  Rutherford  and  Soddy 
in  their  disintegration  theory  of  radio-activity,  and  as  this  view 
accounts  in  a  simple  manner  for  all  the  new  facts  at  present 
known  with  regard  to  the  radio-elements,  and  greatly  assists 
the  presentation  of  the  subject  as  a  connected  whole,  it  'is 
used  as  the  basis  of  arrangement  in  the  remaining  chapters. 
It  must  be  borne  in  mind  that  the  order  in  which  the  researches 
will  be  considered  is  frequently  the  inverse  of  the  historical 
order. 

Uranium,  of  the  three  radio-elements,  furnishes  the  simplest 
example  and  will  be  considered  first.  Sir  William  Crookes 


URANIUM  X   AND    THORIUM   X.  83 

(Proc.  Koy.  Soc.,  1900,  66,  p.  409),  showed  that  it  was  possible 
by  various  processes  to  free  uranium  completely  from  rays 
which  affect  a  photographic  plate,  and  to  concentrate  the  whole 
of  the  photographic  activity  in  a  minute  fraction  of  the  total 
matter  which  contained  no  uranium.  He  gave  several  methods, 
of  which  two  may  be  considered.  If  crystallised  uranium 
nitrate  is  dissolved  in  ether,  the  solution  separates  into  two 
layers:  (1)  an  ethereal  solution  containing  the  bulk  of  the 
dissolved  uranium  salt ;  (2)  anv  aqueous  solution  consisting 
of  the  water  of  crystallisation  of  the  original  salt,  in  which 
a  relatively  small  amount  of  uranium  is  dissolved.  He 
found  that  the  uranium  salt  obtained  from  (1)  was  com- 
pletely inactive  to  the  photographic  plate,  while  that  obtained 
from  (2)  possessed  all  the  activity  ot  the  original  uranium  salt 
in  concentrated  form.  Again,  if  a  solution  of  a  uranium 
salt  be  precipitated  by  ammonium  carbonate  solution,  and  a 
sufficiency  of  the  latter  is  added  in  excess  of  that  necessary  to 
cause  precipitation,  the  precipitated  uranium  carbonate  again 
dissolves,  and  on  filtering  the  solution  a  very  minute  precipitate 
of  insoluble  matter  is  obtained.  This  consists  mainly  of  iron 
and  aluminium  present  in  the  uranium  as  impurities,  and  by 
the  manner  of  its  preparation  is  chemically  free  from  uranium. 
Yet  this  minute  precipitate  possessed  all  the  photographic 
activity  of  the  original  uranium,  and  the  latter  obtained  from 
the  solution  was  completely  inactive.  In  this  way  Crookes 
obtained  preparations  man}'  hundred  times  more  active  to  the 
photographic  plate  than  the  uranium  salt  itself.  Thinking 
that  possibly  he  had  separated  a  new  radio-element  of  intense 
activity,  which  caused  the  radioactivity  of  uranium,  Sir 
William  Crookes  proposed  the  name  "  Uranium  X"  for  the  new 
substance. 

It  must  be  noticed  that  the  actual  amount  of  uranium  X 
obtained  is  probably  quite  incapable  of  detection  except  by  its 
radio-activity.  The  bulk  of  the  substance  is  iron,  aluminium, 
&c.,  present  in  the  uranium  as  minute  impurities  which  afford 
the  necessary  solid  nuclei  for  the  separation  of  the  uranium  X 
by  the  relatively  gross  process  of  filtration.*  No  new  lines 

*  The  Author  has  noticed  that  this  separation  may  fail  if  the  uranium 
preparation  is  too  pure,  until  a  traje  of  ferric  salt  or  similar  impurity  is 
added  to  act  as  nuclei  in  the  nitration. 

G  -2 


84  RADIO-ACTIVITY. 

were  detected  by  Crookes  in  the  spectrum,  and  everything 
points  to  the  conclusion  that  the  actual  amount  of  matter 
causing  the  radio-activity  of  uranium  X  is  infinitesimal. 

Becquerel  ( Comptes  Rendus,  1900,  131,  p.  137),  independently 
of  Crookes,  obtained  evidence  of  the  enfeeblement  of  the  radio- 
activity of  uranium  by  chemical  treatment.  Acting  on  a  sug- 
gestion of  Debierne,  he  dissolved  barium  salts  in  solutions  of 
uranium,  and  precipitated  the  barium  with  sulphuric  acid.  He 
found  that  the  precipitated  barium  sulphate  tended  to  "  drag 
down "  the  activity  of  the  uranium,  leaving  it  more  or  less 
inactive.  He  so  obtained  specimens  of  barium  sulphate  more 
radio-active,  weight  for  weight,  than  the  salts  of  uranium  used 
in  their  preparation. 

The  constancy  of  the  radio-activity  of  uranium,  independent 
of  its  source,  and  previous  history,  which  had  been  shown  by 
Mme.  Curie,  and  was,  indeed,  the  fundamental  starting  point 
of  her  subsequent  discoveries,  caused  Becquerel  to  consider  as 
improbable  Crookes'  hypothesis  that  the  radio-activity  of  uranium 
was  due  to  a  small  intensely  active  impurity  (uranium  X).  He 
concluded  (Comptes  Eendus,  1901,  133,  p.  977)  that,  if  uraniumy 
like  radium,  possessed  the  power  of  making  admixed  inactive 
bodies  temporarily  radio-active  by  association  with  themr 
the  phenomena  could  be  explained.  This  supposed  commu- 
nication of  radio-activity  to  an  inactive  molecule  had  been 
termed  "radio-active  induction."  Thus  M.  and  Mme.  Curie 
found  that  objects  in  the  neighbourhood  of  radium  became 
temporarily  radio-active,  and  called  this  phenomenon  "  the 
induced  activity."  The  theory  of  Rutherford,  which  we  shall 
deal  with  more  fully  in  the  sequel,  that  this  radio-activity  was 
imparted  to  the  inactive  matter  as  a  deposit  of  active  matter 
from  the  radio-active  emanation  of  radium,  had  not  then  been 
accepted  by  the  Continental  investigators.  This  effect  of  radio- 
active induction, Becquerel  argued,  would  be  produced  equally 
on  the  active  and  inactive  matter  present,  so  that  by  induction 
on  itself  the  activity  of  the  mixture  should  increase  spontane- 
ously at  first.  In  a  similar  way  the  activity  of  solid  radium  salts 
was  known  to  increase  from  the  moment  of  their  preparation.  If 
barium  were  rendered  active  by  induction  when  it  was  mixed 
with  uranium,  by  precipitating  the  barium  it  would  be  possible 
to  remove  some  of  the  activity,  and,  then,  after  removal,  the 


URANimr  X  AND   THORIUM  X.  85 

induced  activity  of  the  barium  should  gradually  decay,  in  a 
manner  similar  to  the  induced  activity  of  radium,  while  that 
of  the  uranium  should  gradually  increase.  Becquerel, 
therefore,  examined  his  preparations  18  months  afterwards 
and  found  that  the  activity  of  the  barium  sulphate 
had  completely  disappeared,  while  the  uranium  had  fully 
recovered  its  normal  activity.  Subsequently  it  was  shown 
(Soddy,  Journal  Chem.  Soc.,  1902,  81,  p.  860)  that  the 
processes  of  Crookes  and  Becquerel  resulted  only  in  the 
separation  of  the  /3-radiation  of  uranium,  and  that  the  rela- 
tively far  more  intense  a-radiation  of  uranium  was  completely 
unaffected.  Thus,  when  the  activity  of  the  uncovered  salt  is 
examined  by  the  electrical  method,  it  is  found  to  be  sensibly 
the  same  after  chemical  treatment  as  it  was  before,  although 
the  same  preparation  has  been  rendered  completely  inactive  to 
the  photographic  plate.  On  the  other  hand,  the  uranium  X 
obtained  from  it  by  Crookes'  ammonium  carbonate  process 
was  but  feebly  active  by  the  electrical,  but  intensely  active  by 
the  photographic,  method.  But  a  very  thin  sheet  of  aluminium 
(0'005cm.  thick)  placed  over  the  uranium  completely  absorbed 
the  radiation,  whereas  the  rays  from  uranium  X  are  not 
appreciably  absorbed  by  this  thickness  of  metal.  The  view  that 
the  /3-radiation  of  ordinary  uranium  is  caused  by  uranium  X, 
a  non-uranium  type  of  matter,  which  can  be  chemically 
separated,  whereas  the  a-radiation  is  a  specific  property  of  the 
uranium  and  is  not  affected  by  this  separation,  was  confirmed 
by  Rutherford  and  Grier  (Phil.  Mag.,  1902.,  VI,  4,  p.  315), 
who  examined  the  radiations  from  the  two  preparations  in 
a  magnetic  field.  They  found  the  radiation  from  the  uranium  X 
to  consist  entirely  of  rays  easily  deviated,  while  that  from  the 
uranium  wholly  consisted  of  non-deviable  rays.  The  chemical 
separation  had  analysed  the  radiation  into  its  two  components 
(a  and  (3)  without  affecting  the  nature  or  intensity  of  the  rays 
in  anjr  way.  Uranium  is  the  simplest  example  of  the  three 
radio-elements,  for  its  two  main  types  of  radiation  result 
from  two  different  kinds  of  matter,  the  one  constituting 
practically  the  entire  mass  of  the  substance,  the  other  present 
in  proportion  so  minute  that  its  radio-activity  is  at  present 
the  sole  evidence  we  have  of  its  existence.  The  recovery  of 
the  /^-radiation  of  uranium  with  time,  and  the  decay  of  the 


86  RADIO-ACTIVITY. 

j3-  radiation  from  the  uranium  X  with  time,  thus  appear  to  be 
phenomena  of  momentous  import. 

Simultaneously  with  the  above  researches  Rutherford  and 
Soddy  (Trans.  Chem.  Soc.,  1902,  81,  pp.  321  and  837)  had 
obtained  results  for  the  case  of  thorium  similar  in  character 
to  those  obtained  by  Becquerel  for  uranium.  If  thorium 
nitrate  in  aqueous  solution  is  precipitated  by  ammonia,*  it  is- 
found  that  a  large  part  of  the  radio-activity  of  the  thorium 
remains  in  the  solution,  so  that,  by  evaporating  the  filtrate 
to  dryness  and  igniting  to  remove  ammonium  salts,  minute 
residues  are  left  possessing  radio-activity  relatively  intense 
compared  to  that  of  the  original  thorium.  The  thorium 
hydroxide,  which  is  precipitated  completely  by  this  process,  on 
the  other  hand,  is  found  to  have  lost  the  greater  part  of  its  radio- 
activity, and  by  suitable  repetitions  may  be  obtained  only  one- 
quarter  as  active,  weight  for  weight,  as  the  original  salt.  Now 
this  behaviour  is  by  no  means  due  to  the  precipitation  of 
thorium  from  its  solution.  If  reagents  other  than  ammonia  are 
employed  —  for  example,  sodium  or  ammonium  carbonate, 
ammonium  oxalate  and  alkaline  sodium  phosphate  —  the  thorium 
is  completely  precipitated  in  the  normally  active  condition, 
and  the  residues  obtained  from  the  several  filtrates  by  evapora- 
tion and  ignition  are  in  each  case  quite  inactive. 

Rutherford  and  Soddy,  therefore,  concluded  that  the  major 
part  of  the  radio-activity  of  thorium  is  due  to  the  presence  of 
a  non-thorium  type  of  matter,  not  precipitated  by  ammonia, 
and  they  gave  the  name  "  Thorium  X  "  to  this  body,  in  accord- 
ance with  the  nomenclature  adopted  by  Sir  W.  Crookes  in  the 
case  of  uranium.  Ammonia  is  the  only  reagent  at  present 
known  which  will  separate  thorium  X  from  thorium. 
Besides  being  responsible  for  the  greater  part  of  the  radio- 
activity of  thorium,  the  power  of  thorium  preparations  of 
giving  a  radio-active  emanation  and  of  imparting  radio- 
activity to  surrounding  objects  is  wholly  due  to  the  presence 
of  thorium  X.  For  the  power  of  a  thorium  solution  to  give 
the  emanation  is  retained  unaltered  by  the  solution  after 

*  The  ordinary  chemical  equation  for  this  reaction  is  — 


Thorium       Ammonium       Thorium       Ammonium 
Nitrate  Hydrate         Hydroxide         Nitrate 

(soluble).        (soluble).        (insoluble).       (soluble). 


URANIUM  X  AND   THORIUM  X.  87 

the  thorium  has  been  precipitated  as  hydroxide,  and  the  latter 
when  re-dissolved,  possesses  at  first,  no  emanating  power  at  all. 
As  in  the  case  of  uranium  X,  the  actual  quantity  of  matter 
causing  these  radio-active  manifestations  is,  probably,  infini- 
tesimal. Thorium  X  has  no  definite  analytical  reactions  which 
would  lead  to  its  separation  from  the  impurities  always  present, 
but  is  dragged  down  by  any  precipitate  formed  in  its  solution 
to  a  greater  or  less  extent  by  "adsorption."  Barium  sulphate, 
pre-eminently,  appears  to  possess  the  general  power  of  removing 
in  this  way  the  infinitesimal  traces  of  radio-active  matter 
(ThX,  UrX,  &c.)  present  in  solutions  of  the  radio-elements. 
In  the  language  of  the  theory  of  radio-active  induction  this 
result  would  be  expressed  by  saying  that  barium  is  easilj- 
induced  into  activity  by  contact  with  radio-active  matter. 
The  character  of  the  activity  of  barium,  so  "induced,"  is,  how- 
ever, different  according  as  it  is  precipitated  in  solutions  of 
uranium  or  thorium,  and  in  each  case  is  distinctive  of  the 
original  element.  The  character  of  induced  activity  is  quite 
independent  of  the  nature  of  the  matter  "induced,"  whether 
barium  sulphate  or  any  other  precipitate  which  shows  the 
same  property. 

If  the  preparations  of  thorium  and  thorium  X  so  obtained 
are  allowed  to  remain  untouched,  it  is  found  that  the  thorium 
gradually  regains  the  activity  that  it  has  lost  until  it  is  just 
as  active  as  at  first,  while  the  thorium  X  loses  its  activity 
gradually,  until  it  becomes  at  length  completely  inactive. 
These  processes  occur  rapidly  compared  with  the  similar  case 
of  uranium  and  uranium  X.  In  three  weeks  or  a  month  the 
thorium  X  preparation  has  lost  practically  all  its  radio-activity, 
while  the  thorium  in  the  same  interval  has  completely  regained 
its  lost  radio-activity.  The  same  holds  true  equally  of  the 
emanating  power  of  the  preparations.  The  thorium  X  at  first 
possesses  all  the  emanating  power  of  the  original  thorium 
solution  and  loses  it  completely  in  one  month,  whereas  the 
precipitated  thorium  hydroxide,  if  re-dissolved  in  nitric  acid 
and  kept  in  solution,  at  first  possesses  no  emanating  power  at 
all,  but  gradually  recovers  it,  until,  at  the  end  of  a  month,  it 
again  possesses  its  original  power. 

When  a  thorium  preparation  is  precipitated  in  the  above 
manner  several  times  successively  at  short  intervals,  its  radio- 


RADIO-ACTIVITY. 


activity  is  reduced  to  about  one-fourth  of  the  normal,  and  this 
non-separable  radio-activity,  which  is,  as  far  as  we  know,  a  specific 
property  of  the  element  itself,  consists  only  of  a  rays.  The 
/^-radiation  is  wholly  retained  by  the  thorium  X.  These 
results  may  be  conveniently  represented  in  tabular  form. 


Ordinary  Uranium 

by  addition  of  ammonium  carbonate 
in  excess  to  the  solution 


Ordinary  Thorium 

by  addition  of  ammonia 
in  excess  to  the  solution 


(Insoluble.) 
Uranium  X 

(Soluble.) 
Uranium 

(Soluble.) 
Thorium  X 

(Insoluble.) 
Thorium 

giving 
all  the  /3-rays, 
no  a-rays. 

giving 
all  the  a-rays, 
no  /3-rays. 

giving 
75%  a-rays, 
all  the  /3-rays, 
all  the  emanating 
power. 

giving 

25%  a-rays, 
no  /3-rays, 
no  emanating 
power. 

Thus,  of  the  three  main  phenomena  which  go  to  make  up  the 
normal  radio-activity  of  thorium  compounds  (1)  a-radiation, 
(2) /^-radiation,  (3)  production  of  a  radio-active  emanation,  only 
25  per  cent,  of  the  a-radiation  can  be  considered  to  be  speci- 
fically due  to  the  thorium  atoms,  the  remaining  a-radiation, 
together  with  the  /^-radiation  and  the  emanating  power,  being 
caused  by  the  presence  of  a  non- thorium  type  of  matter — 
thorium  X — present  in  infinitesimal  quantity.  Whereas,  how- 
ever, the  thorium  X,  on  keeping,  rapidly  loses  its  radio- 
activity, the  thorium  from  which  it  was  separated  regains  its 
lost  radio-activity,  until  at  the  end  of  a  month  it  again 
possesses  its  normal  or  maximum  value.  In  the  same  time 
the  thorium  X  becomes  completely  inactive.  If,  now,  the 
thorium  that  has  completely  recovered  its  radio-activity  is 
again  precipitated  with  ammonia,  a  new  amount  of  thorium  X 
is  obtained  from  the  filtrate  of  the  same  radio-activity  as  that 
originally  extracted,  and  the  thorium  is  again  freed  from  the 
major  part  of  its  activity.  This  cycle  of  operations  can  be 
repeated  indefinitely.  If,  however,  the  precipitations  are 
carried  out  without  lapse  of  time,  the  radio-activity  of  the 
thorium  is  not  further  reduced  below  a  certain  minimum  value, 
and  when  this  is  reached  no  more  thorium  X  is  obtained  and 
the  residues  from  the  filtrates  are  inactive.  By  waiting  for 


URANIUM  X  AND    THORIUM  X.  89 

any  period  less  than  one  month  between  successive  precipita- 
tions a  new  quantity  of  thorium  X  is  obtained,  in  amount  (as 
judged  by  its  activity,  which  is  the  only  criterion  we  have  of 
its  presence)  the  greater  the  longer  the  time  waited. 

If  it  is  admitted  that  thorium  X  is  a  specific  kind  of  matter, 
the  only  explanation  of  this  behaviour  is  that  it  is  being 
continuously  produced  by  thorium.  This  is  equivalent  to  saying 
that  thorium  is  changing  into  thorium  X,  unless  we  suppose  that 
thorium  has  the  power  of  creating  thorium  X,  which  is  hardly 
conceivable  in  the  present  state  of  science.  The  view  that 
thorium  X  is  a  specific  kind  of  matter  therefore  necessi- 
tates the  belief  that  an  element  is  capable  of  undergoing  a 
slow  spontaneous  change,  and  it  will  be  convenient  to  desig- 
nate this  by  the  special  name  of  radio-active  cliange.  As  this  is 
the  point  of  departure  from  which  springs  the  whole  theory 
of  atomic  disintegration  as  applied  to  explain  radio-activity, 
it  is  necessary  to  bear  in  mind  the  precise  character  of 
the  fundamental  proposition.  The  alternative  is  to  suppose 
that  thorium  X  is  not  a  specific  kind  of  matter,  but  is 
ordinary  inactive  matter  of  some  kind  unknown,  in  which 
.strong  radio-activity  has  been  "  induced  "  by  association  with 
the  thorium,  and  which  loses  again  this  property  when  it  is 
removed  from  the  thorium.  The  evidence  in  favour  of  the 
first  and  opposed  to  the  second  view  is  drawn  not  only  from 
the  behaviour  of  thorium  X  and  uranium  X,  but  from  the  beha- 
viour of  at  least  six  other  similar  types  of  temporarily  radio- 
active matter.  The  most  conclusive  evidence  is  drawn  from  the 
study  of  the  radio-active  emanations  of  thorium  and  radium, 
which  have  still  to  be  considered.  On  the  other  hand,  the 
second  view  finds  to-day  no  support,  and  no  attempt  has 
been  made,  since  the  processes  of  radio-activity  have  become 
better  understood,  to  reconcile  the  idea  of  radio-active  "  induc- 
tion "  with  the  overwhelming  evidence  that  can  now  be  brought 
against  it.  The  effects  to  which  the  term  "induction"  was  first 
applied  are  now  admitted  to  be  capable  of  a  direct  explanation, 
and  the  process,  in  the  sense  of  the  communication  of  radio- 
activity to  an  inactive  molecule,  has  never  been  shown  to  take 
place.  Even  if  it  did,  it  would  merely  constitute  an  additional 
fact  to  be  explained  without  itself  aiding  in  the  explanation  of 
any  of  those  already  known. 


90 


RA  DIO-A  GTIVIT  Y. 


Once,  however,  the  possibility  of  an  element  undergoing 
change  is  admitted,  the  varied  phenomena  of  radio-activity  can 
be  explained  from  a  consistent  point  of  view  without  further 
difficulty. 

A  quantitative  study  of  the  rate  of  recovery  of  the  activity 
of  thorium  and  the  rate  of  decay  of  the  activity  of 
thorium  X  (Rutherford  and  Soddy,  Phil.  Mag,  1902,  VL,  4, 
p.  378)  revealed  a  simple  connection  between  the  two  pro- 
cesses. Fig.  25  represents  the  curves  obtained  in  which  the 
radio-activity  is  plotted  on  the  vertical  and  the  time  in  days 
on  the  horizontal  axis.  Curve  I.  represents  the  decay  of 
120 


no 


2         46         8        10       12       14      16       18        20     22 
Time  in  Days. 

FIG.  25. 

activity  of  thorium  X,  the  initial  activity  being  represented 
as  100,  and  curve  II.  the  recovery  of  activity  of  thorium 
to  a  constant  maximum  value,  represented  also  as  100. 
For  the  first  day  the  effects  are  abnormal,  the  activity 
of  the  thorium  X  increasing  slightly  before  commencing 
to  diminish,  while  that  of  the  thorium  at  first  slightly 
diminishes  and  then  proceeds  to  increase  in  a  regular  manner. 
These  initial  irregularities  will  be  examined  later.  For  the 


URANIUM  X  ASD   THORIUM  X. 


91 


present  they  may  be  neglected,  and  the  subsequent  regular 
changes  be  considered,  without  involving  any  serious  error.  It 
will  be  noticed  that  the  recovery  curve,  if  produced  backwards, 
cuts  the  vertical  axis  at  about  25  per  cent.,  and  this  can  be 
shown  to  be  due  to  the  thorium  possessing  a  constant  non- 
separable  radio-activity  specific  to  the  element  itself.  In  Fig.  26 
the  relation  between  the  two  curves  is  more  clearly  brought 
out.  Curve  I.  represents  the  percentage  proportion  of  the 
activity  recovered  by  the  thorium  after  the  second  day,  the 
activity  recovered  from  the  beginning  to  the  end  being  taken 
as  100.  Curve  II.  represents  the  activity  of  thorium  X  on  the 
same  scale  during  the  same  period. 


2       4 


8      10    12     14    16    18     20      22    24    26 
Time  in  Days. 
FIG.  26. 


It  will  be  seen  that  the  proportion  of  the  radio-activity 
recovered  by  the  thorium  during  any  interval  throughout  the 
whole  period  is  equal  to  the  proportion  of  the  radio-activity 
lost  by  the  thorium  X  during  the  same  interval.  If  I0 
represents  the  original  activity  ot  the  thorium  X,  and  lt  the 
activity  after  any  time  /,  I't  the  activity  recovered  by  the 
thorium  after  time  t,  and  I'  «>  the  maximum  activity  recovered 
by  the  thorium  when  the  constant  value  is  attained, 

I't-1""1*-!  _*«  (1) 

TV  f  X         T    '     *         '  *        •        \    / 

1  oo  X0  % 

The  activity  of  the  thorium  X  decays  very  approximately 
in  a  geometrical  progression  with  the  time,  falling  to  half-value 


92  11  AD  10-  ACTIVITY. 

in  four  days.  Thus,  in  eight  days  it  is  one-quarter  of  the 
original,  in  12  days  one-eighth,  and  so  on.  This  is  expressed 
by  the  equation 


where  A  is  a  constant  and  e  the  base  of  natural  logarithms. 

Since  ^  =  J  when  £  =  4  days  =  345,600  seconds,  A  =  2xlO~6, 

-Lo 
when  t  is  expressed  in  seconds. 

From  equations  (1)  and  (2)  we  can  write  at  once 


(3) 


which  represents  the  recovery  of  activity  by  the  thorium  with 
time.  The  law  of  the  decay  of  activity  of  thorium  X  in  a 
G.P.  with  the  time  (equation  2)  is  a  general  law  for  tempo- 
rarily radio-active  substances.  Many  such  substances  are 
known,  decaying  at  characteristic  rates,  varying  from  a  few 
seconds  to  a  few  years,  but  always  according  to  the  same  law. 
Moreover,  the  rate  of  decay  has  been  found  to  be  independent 
of  the  most  powerful  chemical  and  physical  agencies.  Extremes 
of  temperature  and  drastic  chemical  treatment  do  not  at  all 
affect  it.  In  consequence,  A  (equation  2)  serves  as  a  perfectly 
definite  constant  for  the  identification  and  characterisation 
of  any  of  these  new  bodies,  none  of  which  have  been 
obtained  in  sufficient  quantity  for  the  ordinary  methods 
of  investigation.  A  may  be  called  the  radio-active  constant, 
and  we  shall  see  later  that  it  possesses  a  profound  physical 
meaning. 

The  chemical  separation  of  thorium  X  from  thorium,  there- 
fore, cannot  have  any  effect  on  the  activity  of  the  former 
in  causing  it  to  decay.  The  activity  of  thorium  X  must 
always  be  decaying  at  the  same  rate,  whether  it  is  present 
in  the  thorium  compound  producing  it  or  whether  it  is 
separated  from  it.  The  apparent  constancy  of  activity  of  the 
radio-elements  results  from  the  continuous  production  of 
fresh  matter  possessing  continuously  diminishing  radio-activity. 
When  the  increase  in  radio-activity  through  the  production  of 
new  active  matter  balances  the  decrease  through  the  decay 
of  the  activity  of  that  already  formed,  the  activity  remains 


URANIUM  X  AND    THORIUM  X.  93 

constant.  This  point  may  be  termed  the  radio-active  equilibrium. 
The  mystery  of  the  constancy  and  permanence  of  the  emission 
of  rays  from  radio-active  substances,  which  from  the  first 
riveted  attention  on  the  new  property,  is,  therefore,  one  step 
nearer  elucidation.  The  rays  in  question  are  always  dying 
down  and  are  always  being  renewed.  The  constancy  is 
apparent  rather  than  real,  and  is  due  to  the  balance  or 
equilibrium  between  opposed  processes. 

The  next  consideration  that  follows  from  equations  (2)  and 
(3)  is  that  a  uniform  quantity  of  thorium  X,  as  measured  by 
its  radio  activity,  is  produced  in  each  unit  of  time.  The 
constancy  of  the  activity  after  radio-active  equilibrium  is 
attained  shows  that  the  production  of  new  matter  must  proceed 
at  a  constant  rate.  This  holds  true,  not  only  for  the  condition 
of  radio-active  equilibrium,  but  generally.  Thus,  equation  (3), 
which  was  obtained  as  the  experimental  relation  holding  for  the 
recovery  of  activity  of  thorium,  is  that  theoretically  required 
for  the  rise  of  activity  of  a  system  in  which  (1)  the  rate  of 
supply  of  fresh  radio-active  matter  proceeds  at  a  uniform  rate  ; 
(2)  the  radio-activity  of  the  matter  supplied  decays  with  time, 
according  to  equation  (2)  (compare  Rutherford,  Phil.  Mag.r 
1900,  V.,  49,  p.  179). 

The  form  of  the  recovery  curve  is  not  altered  by  the  con- 
ditions under  which  the  change  occurs.  It  was  found  that 
different  parts  of  the  same  specimen  of  thorium,  which  had 
been  freed  from  thorium  X,  kept  under  widely  different  con- 
ditions of  temperature,  state  of  chemical  combination,  &c., 
recovered  their  radio-activity  at  the  same  rate.  This  absolute 
independence,  so  far  as  we  know  at  present,  of  the  rates  of 
recovery  and  decay,  towards  all  the  agencies  at  our  disposal, 
is  one  of  the  most  striking  features  of  radio-active  change. 
In  this  respect  the  latter  is  sharply  differentiated  from  all  the 
other  kinds  of  material  change,  whether  physical  or  chemical. 
The  process  which  proceeds  spontaneously  in  Nature,  and 
gives  rise  to  the  phenomenon  of  radio-activity,  is  entirely 
beyond  the  range  of  ordinary  molecular  forces.  This  of 
itself  furnishes  strong  presumptive  evidence  in  favour  of  the 
view  that  radio-active  change  involves  an  alteration  in  the 
internal  structure  of  the  chemical  atom  and  its  transmutation 
into  other  atoms.  For.  since  molecular  forces  are  quite  incapable 


94  RADIO- A  CT1 V1T  Y. 

of  transmuting  one  elementary  form  of  matter  into  others, 
it  is  not  to  be  expected  that  such  forces  would  produce  any 
effect  on  the  course  of  a  natural  process  of  transmutation 
which  was  proceeding  spontaneously.  For  a  similar  reason  it 
is  extremely  improbable  that  any  such  effect  as  radio-active 
induction,  or  the  rendering  of  an  inactive  substance  radio- 
active, exists.  If  it  did  it  would  constitute  a  case  of  artificial 
transmutation,  which,  at  least,  requires  further  proof  before 
being  accepted. 

On  the  new  views,  the  source  of  the  energy  dissipated  by 
radio-active  substances  no  longer  presents  any  fundamental 
difficulty.  The  explanation  is  similar  in  kind  and  differs  only 
in  degree  from  the  well-known  cases  of  slow  spontaneous 
chemical  changes  in  which  energy  is  liberated,  as,  for  example, 
in  the  ordinary  processes  of  living  organisms.  In  each  case 
matter  is  "  consumed,"  that  is,  transformed  into  new  kinds 
which  possess  less  energy.  Direct  experimental  data,  to  be 
considered  in  the  sequel,  show  that  the  energy  of  radio-active 
change  is  of  the  order  of  a  million  times  greater  than  is  ever 
manifested  in  ordinary  chemical  change.  It  is  in  the  enormous 
stores  of  energy  liberated  during  radio-active  change  that  the 
disintegration  theory  derives  its  most  unanswerable  argument. 
A  fuller  consideration  of  this  aspect  of  the  question  must  wait 
until  the  case  of  radium  has  been  considered.  It  is  probable 
that  the  order  of  the  energy  liberated  is,  for  equal  iveiglits  of 
matter  changing,  similar  in  all  the  radio -elements.  But  in  the 
case  of  radium  the  change  proceeds  a  million  times  faster  than 
in  the  case  of  uranium  or  thorium,  and  this  accounts  for  the 
surprising  nature  of  the  properties  of  the  first-named  element. 

An  examination  of  the  recovery  and  decay  of  the  radio- 
activity of  uranium  and  uranium  X  (Rutherford  and  Soddy, 
Phil  Mag.,  1903,  VI,  5,  p.  422)  showed  that  the  considera- 
tions just  developed  for  thorium  apply  equally  well  for  the 
•case  of  uranium.  Nearly  six  months  is  required,  however, 
before  the  radio-active  equilibrium  is  attained  in  this  case. 
Since  the  uranium  X  gives  all  the  /3  or  penetrating  rays  of 
uranium,  the  course  of  the  production  of  uranium  X  can  be 
followed  by  measuring  the  penetrating  radiation  from  uranium 
after  the  uranium  X  has  been  separated  chemically.  Fig.  27 
shows  the  curves  of  recovery  and  decay  of  the  penetrating 


URANIUM  X  AND    THORIUM  X. 


95 


radiation  of  uranium  and  uranium  X  with  time.  When 
separated  from  the  uranium  producing  it,  the  activity  of 
uranium  X  decays  in  a  geometrical  progression  with  the  time, 
as  shown  in  Fig.  27,  and  falls  to  half-value  in  about  22 
days.  The  value  of  A  (equation  2)  for  this  case  is,  there- 
fore, about  3*6  x  10~7,  when  t  is  expressed  in  seconds. 
When  free  from  uranium  X,  uranium  gives  practically  no 
penetrating  rays.  The  gradual  recovery  of  the  /^-radiation  by 
the  uranium  is  shown  by  the  second  curve  of  Fig.  27.  The 
proportionate  loss  of  activity  of  uranium  X  is  for  any  time 
approximately  equal  to  the  proportionate  recovery  of  the 
/3  activity  of  the  uranium,  and  equation  (3),  with  the  value  of 
A  just  given,  applies  to  the  recovery  of  the  activity  of  uranium 
equally  to  that  of  thorium. 


10  20  30  40  50  60  70  80  90  100  110  120  130 140 150 160 
Time  in  Days. 

FIG.  27. 


Special  interest  attaches  to  the  case  of  uranium  on  account 
of  its  simplicity.  The  radiation  can  be  analysed  into  its 
two  constituents  and  the  uranium  shown  to  be  responsible 
for  the  whole  of  the  one  type,  the  uranium  X  for  the 
whole  of  the  other.  Then,  again,  uranium,  unlike  thorium 
and  radium,  does  not  produce  radio-active  emanations  or 
impart  radio-activity  to  its  surroundings.  These  secondary 
effects  cause  the  initial  irregularities  in  the  curves  of  decay 
and  recovery  for  thorium  which  are  absent  in  the  case  of 
uranium.  On  the  other  hand,  the  non-separable  activity, 
which  in  the  case  of  thorium  is  a  relatively  small  fraction  of  the 


96  RADIO-ACTIVITY. 

whole,  in  the  case  of  uranium  comprises  the  whole  of  the  a-radia- 
tion,  and  therefore  the  greater  part  of  the  total  energy  radiated. 
The  existence  of  this  non  separable  radio-activity  is  general  for 
all  three  radio-elements,  and  does  not  appear  to  be  merely  due 
to  the  insufficiency  of  our  chemical  methods  in  analysing  the 
sources  of  the  radio-activity.  It  is  possible  that  further  chemical 
investigation  will  show  that  what  is  here  termed  non-separable 
activity  is  in  reality  due  in  part  to  new  types  of  matter  not 
yet  separated  from  the  radio-elements.  But  it  is  unlikely  that 
the  latter  will  ever  be  obtained  by  any  chemical  process  entirely 
free  from  radio-activity,  and  then  recover  it  with  the  lapse 
of  time.*  In  fact,  the  non- separable  activity  has  a  very 
important  and  fundamental  bearing  on  the  final  view  we  adopt 
of  the  cause  and  nature  of  radio-activity. 

Two  observers  (Brauner,  Trans.  Chem.  Soc.,  1898,  VoL 
LXXIIL,  p.  951 ;  Baskerville,  Journ.  Am.  Chem.  Soc.,  1901, 
XXIIL,  p.  761),  have  obtained  evidence  of  a  new  element  of 
heavier  atomic  weight,  associated  with  thorium  in  its  com- 
pounds, and  the  latter  believes  that  pure  thorium  compounds 
are  not  radio-active.  Recently,  Hoffman  and  Zerban  (Bericlite 
der  Deutschen  Cliemischen  Gesellschaft,  1903,  p.  3,093)  state  that 
the  thorium  prepared  from  gadolinite  is  not  radio-active. 
Before  these  results  can  be  accepted,  more  details  as  to  the 
method  of  testing  and  the  time  since  preparation  of  the 
specimens  employed  must  be  made  known.  For  both 
Becquerel  (Comptes  Rendus,  1902,  134,  p.  208)  and  Sir 
William  Crookes  (Proc.  Roy.  Soc.,  1900,  Vol.  LXVL,  p.  409) 
considered  that  they  had  separated  the  radio-activity  from 
uranium,  although  we  know  now  that  this  result  was  only 
partially  true  (compare  Trans.  Chem.  Soc.,  190::,  Vol. 
LXXXL,  p.  860).  The  non-activity  of  thorium  from  gado- 
linite has  already  been  called  into  question  (George  F.  Barker, 
Am.  Joun.  Science,  1903  [4],  16,  p.  161),  and  reaffirmed 

*  It  is,  of  course,  possible  that  what  we  call  thorium  and  uranium 
respectively  may  prove  to  be  mixtures  of  more  than  one  element,  only 
one  of  which  is  radio-active.  In  this  case  the  proportion  of  the  respective 
active  constituents  must  be  the  same  in  all  preparations  of  these  elements 
that  have  been  examined,  for  they  all  possess  similar  activity.  Moreover, 
on  separating  the  active  constituent  the  uranium  and  thorium  will  then 
not  recover  their  lost  activity. 


URANIUM  X  AND   THORIUM  X.  97 

(Zerban,  Ber.,  1903,  p.  3,911).  Even  if  it  is  ultimately 
established  it  does  not  interfere  with  the  above  reasoning,  for 
when  once  the  active  constituent  is  separated  from  thorium 
the  latter  will  not  recover  its  activity  with  time.  The  con- 
siderations deduced  with  regard  to  the  non-separable  activity 
would  then  apply  without  modification  to  the  active  con- 
stituent, and  whether  this  is  thorium  or  so  like  thorium  that 
it  has  not  yet  been  separated  from  it,  is  a  matter  of  little 
theoretical  importance  in  the  present  connection. 


CHAPTEK  VII. 


THE  EADIO-ACTIVE  EMANATION  OF  THORIUM. 

The  Variability  of  the  Radio-activity  of  Thorium. — Effect  of  Air-currents. 
—  Uranium  and  Thorium  Contrasted. — The  Eadio-active  Emana- 
tion of  Thorium.  —  Emanating  Power  Proportional  to  Weight, 
Radiating  Power  to  Surface. — The  Radiation  of  the  Emanation 
consist  of  only  a-Rays. — Rate  of  Decay  of  Activity. — The  Emanation 
Analogous  to  the  Argon  Family  of  Elements. — Decay  of  Activity 
Unaffected  by  Temperature,  <&c. — Emanating  Power  Persists  in  Absence 
of  the  Atmosphere.— Emanation  Condensed  by  Liquid  Air. — Produced 
by  Radio-active  Change  of  Thorium  X.— Imparted  Radio-activity. — 
Due  to  Matter  Deposited  from  the  Emanation. — Rate  of  Decay. — 
Imparted  Activity  Concentrated  by  an  Electric  Field. — Produced  by 
Radio-active  Change  of  Emanation. 

A  detailed  investigation  of  the  radiation  from  thorium 
(Owens,  Phil.  Mag.,  1899,  V.,  48,  p.  360)  brought  to  light  a 
curious  difference  between  the  radio-activity  of  this  element 
and  that  of  thorium.  It  was  found  that  the  ionisation 
current  through  the  air  in  a  closed  space  under  the  influence 
of  the  rays  from  the  various  thorium  compounds  increased 
with  time  up  to  a  maximum  value.  If  air  was  then  drawn 
continuously  through  the  apparatus  by  means  of  a  pump,  the 
current  immediately  decreased  to  a  minimum  value.  This 
effect  was  more  marked  for  a  thick  layer  of  thorium  com- 
pound than  for  a  thin  layer.  In  the  former  case  the 
maximum  current  was  three  times  as  great  as  the  minimum. 
Rutherford  (Phil  Mag.,  1900,  V.,  49,  p.  1)  showed  that  this 
effect  was  due  to  thorium  compounds  possessing  the  property 
of  continuously  emitting  into  the  air  particles  of  some  kind 
which  possessed  temporary  radio  activity.  That  is,  the  air  in 
the  neighbourhood  of  a  thorium  compound  possessed  the  power 


100  RADIO-ACTIVITY. 

of  giving  out  radiations  on  its  own  account,  and  this  power 
persisted  in  the  air  for  some  time  after  the  thorium  compound 
was  withdrawn.  Rutherford  named  the  radio-active  substance 
communicated  to  the  air  in  this  way  the  "thorium  emanation," 
and  although  it  is  now  known  to  be  a  distinct  type  of  matter, 
like  thorium  X  or  uranium  X,  the  original  name  has  been 
retained.  He  found  that  the  rays  of  thorium  are  completely 
absorbed  by  covering  the  salt  with  a  single  thickness  of  foolscap 
paper,  whereas  the  emanation  readily  finds  its  way  through  20 
layers,  so  that  the  effect  of  the  emanation  alone  can  be  well 
studied  by  enclosing  the  thorium  salt  in  paper.  Under  these 
circumstances  the  ionisation  current  observed  when  the  package 
is  placed  between  the  plates  of  the  testing  apparatus  is  reduced 
to  a  very  small  fraction  of  its  original  value  when  the  blast  of 
air  is  directed  between  the  plates.  The  effect  of  /3-rays  of 
thorium  in  experiments  by  the  electrical  method  is  exceedingly 
small  and  would  hardly  be  detected  except  by  special  arrange- 
ments. The  thinnest  sheet  of  mica  or  glass  is  completely 
impervious  to  the  passage  of  the  emanation.  In  this  and  all 
other  respects  in  which  it  has  been  examined  the  emanation 
behaves  like  a  gas  distributed  in  infinitesimal  amount  through 
the  atmosphere  which  carries  it,  each  particle  emitting  rays  on 
its  own  account  and  acting  as  a  centre  of  ionisation  in  the 
atmosphere. 

The  "  emanating  power,"  or  ability  to  give  the  radio- 
active emanation,  is  proportional  to  the  quantity  of  the 
thorium  compound  employed,  whereas  the  ladiating  power  is 
proportional  to  the  surface  exposed.  Thus,  in  very  thin  layers 
the  ionisation  is  caused  chiefly  by  the  radiation,  and  as  the 
layer  is  increased  in  thickness  the  additional  ionisation  due 
to  the  emanation  is  increased.  Hence,  in  the  former  case  a 
blast  of  air  has  little  effect  on  the  value  of  the  radio-activity. 
Thin  layers  of  thorium  preparations  are  in  this  respect  little 
different  from  those  of  uranium.  With  thicker  layers  the 
difference  is,  however,  very  marked,  and  the  value  of  the 
radio-activity  is  greatly  affected  by  slight  currents  of  air. 
All  these  phenomena  are  quite  independent  of  the  nature  of 
the  atmosphere  employed. 

The  properties  of  the  radio-active  emanation  itself  will  be 
first  considered,  without  reference  to  the  manner  in  which  it 


RADIO-ACTIVE  EMANATION  OF  THORIUM.         101 

comes  to  be  produced  by  the  thorium.  The  experiments 
quoted  show  that  thorium  transmits  to  the  air  an  ionising 
agent  with  the  properties  of  a  gas,  but  the  question  remains 
open  as  to  whether  the  ionisation  is  produced  by  rays  of  the 
kind  which  constitute  radio-activity.  This  is  a  difficult  point 
to  establish  experimentally  in  the  case  of  the  thorium  emanation, 
and  was  accomplished  by  Rutherford  in  the  fpllowing  manner* 
A  vessel  was  constructed  through  which  th«  thorium  emanation 
could  be  led  by  a  current  of  air.  In  one  si$e  of  the -vessel  was  ,a 
window  of  the  thinnest  mica  sheet,  whicn,  although -gas -tight 
and  impervious  to  the  passage  of  the  emanation,  was  thin 
enough  to  allow  a  sufficient  fraction  of  a-radiation  of  ordinary 
penetrating  power  to  pass  through  unabsorbed.  The  effects 
of  the  imparted  activity  (q.v.)  were  eliminated  by  keeping  the 
vessel  positively  charged  with  respect  to  an  internal  electrode 
placed  out  of  the  direct  line  of  fire  to  the  mica  window.  In  this 
way  any  imparted  activity,  produced  by  the  emanation,  was 
concentrated  on  the  negative  electrode,  the  radiations  from 
which  were  unable  to  penetrate  to  the  outside  of  the  vessel. 
Under  these  circumstances  the  outside  air  in  the  line  of  fire 
from  the  mica  window  was  found  to  be  ionised  by  rays  which 
could  only  come  from  the  gaseous  emanation  inside  the  vessel. 
Rutherford  proved  with  this  arrangement  that  the  rays  from 
the  thorium  emanation  consist  entirely  of  the  easily-absorbed 
and  difficultly-deviable  a-rays,  and  no  detectable  amount  of 
/3-rays  are  present.  This  experiment,  difficult  for  the  thorium 
emanation,  has  been  shown  in  the  simplest  manner  for  the 
millionfold  more  powerful  radium  emanation  (q.v.).  The 
emanation  particles  do  not  appear  to  move  appreciably  under 
the  action  of  an  electric  field,  and  Rutherford  concluded  that 
they  are  uncharged.  Our  present  knowledge  shows  that  this 
result  proves,  rather,  that  if  the  emanation  particle  is  charged 
it  can  give  up  its  charge  without  losing  either  of  its  essential 
characteristics — viz.,  its  volatility  or  radio-activity.  The  point 
will  be  again  referred  to  when  the  actinium  emanation  is  dealt 
with. 

Rate  of  Decay  of  the  Thorium  Emanation. — If  the  emanation  is 
blown  away  from  the  thorium  by  a  blast  of  air  into  a  metal 
cylinder  (which  can  be  closed  air-tight)  provided  with  a  central 
insulated  wire,  the  duration  of  its  radio-activity  can  be  inves- 


102  RADIO-ACTIVITY. 

tigated  by  measuring  the  ionisation  current  between  the  central 
wire  and  the  cylinder,  after  the  air  stream  is  stopped  and 
the  cylinder  closed.  In  this  way  Kutherford  showed  that  the 
radio-activity  of  the  emanation,  after  separation  from  the 
thorium  compound,  decays  in  a  geometrical  progression  with 
the  time,  and  falls  to  half-value  in  about  one  minute.  This 
is ;  expressed  tbyt  the  equation  If/I0  =  e~xi,  in  which  the  radio- 
active con-stanfc^A  ^h&s  a  value  of  1/87,  or  l'16xlO~2.  In 
}Q  ^minutes  ajfrer;,  tjj£  removal  of  the  emanation  from  the 
t&driuin  the  radioactivity  falls  to  one-thousandth  of  its  original 


*~  -1'h 


Earth 
FIG.  28. 

value,  a  quantity  generally  too  small  to  be  detected.  In 
20  minutes  the  activity  is  one-thousandth  of  the  value  at 
10  minutes  or  one-millionth  of  the  initial. 

A  convenient  apparatus  for  the  study  of  the  thorium  emana- 
tion is  shown  in  Fig.  28.  A  known  weight  of  the  thorium 
compound  to  be  tested  is  placed  in  C,  and  a  steady  current  of 
air  from  a  gas  bag  passed  over  it.  The  cotton  wool  in  D 
removes  from  the  air  the  ions  caused  by  the  action  of  the  direct 
radiation.  The  emanation  passes  through  the  plug  and  causes 
the  ionisation  in  the  cylinder.  With  a  uranium  compound  in 
C  no  current  would  be  observed  with  this  apparatus,  for  all 
the  ions  present  in  the  gas  would  be  removed  on  passing 
through  D.  With  10  grammes  of  ordinary  thorium  oxide  in  C, 
and  a  fairly  rapid  stream  of  air,  a  current  between  10~10  and 
10~n  ampere  passes  between  the  cylinder  and  the  electrodes. 
The  central  electrode  is  divided  into  three  separate  parts,  E,F,  H, 
for  the  purpose  of  determining  the  rate  of  decay  of  the  emana- 
tion. If  the  velocity  of  the  gas  stream  is  known,  the  ionisation 


RADIO-ACTIVE  EMANATION  OF  THORIUM.         103 

currents  respectively  measured  by  connecting  the  three  elec- 
trodes successively  with  the  electrometer  will  afford  a  measure 
of  the  decay  of  activity  suffered  by  the  emanation  in  the  time 
taken  for  the  gas  stream  to  pass  from  E  to  F  and  from  F  to 
H.  If  the  gas  stream  is  kept  constant,  the  rates  of  decay  of 
the  activity  of  the  emanations  from  different  compounds  of 
thorium,  and  from  the  preparations  of  thorium  X,  can  be  com- 
pared together.  It  is  found  that  the  rate  of  decay,  and  there- 
fore the  radio-active  constant  A,  is  the  same  in  all  cases.  In 
ordinary  work  when  the  rate  of  decay  is  not  being  investi- 
gated the  three  electrodes  E,  F,  H,  are  usually  connected 
together. 

The  thorium  emanation  was  the  first  type  of  matter  pos- 
sessing temporary  radio-activity  to  be  recognised,  and  its 
nature  had  been  studied  pretty  completely  before  thorium  X 
was  discovered.  In  this  work  (Rutherford  and  Soddy,  Phil. 
Mag.,  1902,  VI.,  4,  p.  569)  the  conclusion  was  drawn  that  it 
was  a  specific  type  of  radio-active  matter  of  a  gaseous  character 
in  infinitesimal  quantity.  The  latter  presents  no  drawback 
to  its  accurate  investigation,  for  its  radio-activity  affords 
adequate  and  convenient  methods  for  its  experimental  study. 
Various  experiments  were  performed  on  the  action  of  tem- 
perature and  chemical  reagents  on  the  emanation.  The 
general  result  showed  that  it  was  completely  unaffected  by 
any  of  the  means  employed.  It  is  not  altered  by  passage 
through  a  tube  of  platinum  raised  to  the  temperature  of 
bright  white  heat.  It  may  be  bubbled  unchanged  through 
any  acid  or  reagent.  It  is  not  absorbed  by  passing  through 
red-hot  lead  chromate,  magnesium  powder,  zinc  dust,  &c., 
and  in  this  respect  it  shows  the  same  chemical  inertness  as 
the  members  of  the  argon  family  of  gases.  The  latter,  and 
the  emanations  of  thorium  and  radium,  are  the  only  known 
gases  which  resist  absorption  by  these  reagents.  The  appa- 
ratus employed  in  these  experiments  was  that  already  described 
(Fig.  28),  the  tubes  containing  the  reagents  used  being  placed 
in  the  gas  stream  between  D  and  the  testing  cylinder.  A  steady 
stream  of  gas  was  maintained  and  the  ionisation  current  mea- 
sured with  the  tubes  hot  and  cold.  Since  the  value  of  the 
ionisation  current  was  not  altered  by  this  treatment,  it  not 
only  proves  that  the  emanation  is  not  absorbed,  but  also  that 


104  RADIO-ACTIVITY. 

the  rate  of  decay  of  the  activity  of  the  emanation  is  the  same 
throughout,  and  is  not  affected  either  by  temperature  or  by 
chemical  reagents. 

Some  experiments  were  performed  by  the  Author  with  a 
view  to  determine  if  the  presence  of  the  air  were  necessary  for 
the  production  of  the  emanation  by  thorium.  An  apparatus 
for  producing  hydrogen  and  oxygen  by  the  electrolysis  of 
dilute  sulphuric  acid  was  constructed,  and  the  gas  passed  over 
a  thorium  compound.  All  the  joints  were  of  sealed  glass,  so 
that  there  was  no  possibility  of  air  leaking  into  the  apparatus. 
Over  a  thousand  litres  of  hydrogen  were  passed  over  the 
thorium  compound,  without  opening  the  apparatus  to  the  air, 
in  an  uninterrupted  stream  lasting  for  three  months.  At  the 
end  of  this  period  the  amount  of  emanation  carried  over  by 
the  gas  stream  was  no  less  than  at  the  beginning.  Hence  the 
emanation  is  not,  for  example,  one  of  the  inert  constituents 
of  the  atmosphere  rendered  radio-active  by  contact  with  thorium, 
and  there  is  no  escape  from  the  conclusion  that  the  emanation 
is  a  specific  type  of  matter  of  a  new  kind  derived  from  the 
element  thorium.  This  conclusion  was  confirmed  by  an  inves- 
tigation of  the  behaviour  of  the  emanations  of  thorium  and 
radium  at  the  low  temperatures  attainable  by  the  use  of  liquid 
air  (Rutherford  and  Soddy,  Phil.  Mag.,  1903,  VI,  5,  p.  561).  It 
was  found  that  the  emanations  of  both  thorium  and  radium 
were  completely  condensed  if  passed  through  a  tube  cooled 
with  liquid  air.  The  gas  stream  used  to  convey  it  passes 
on  completely  freed  from  the  emanation.  If  the  tube  is 
allowed  to  warm  up,  the  condensed  emanation  again  volatilises 
unchanged  (compare  Chapter  X.). 

Now,  the  experiments  already  recorded  on  thorium  X 
show  that  the  emanation  is  not  produced  by  thorium  directly, 
for  when  freed  from  thorium  X  the  thorium  possesses  no 
emanating  power  at  all  at  first,  but  gradually  recovers  it.  The 
thorium  X,  on  the  other  hand,  possesses  the  full  emanat- 
ing power  of  the  original  substance  from  which  it  was 
separated,  and  gradually  loses  its  power  as  the  thorium 
recovers  it. 

Without,  at  the  present  stage,  considering  more  fully  the 
nature  of  these  phenomena,  a  simple  explanation  may  be 
developed  on  the  lines  laid  down  in  the  discussion  of  thorium  X. 


RADIO-ACTIVE  EMANATION  OF  THORIUM.         105 

It  is  clear  that,  just  as  thorium  X  must  be  considered  as  the 
product  of  the  radio-active  change  of  thorium,  so  the  emanation 
in  turn  must  be  regarded  as  the  product  of  a  second  change  of 
a  similar  kind  suffered  by  thorium  X.  Thorium  is  changing 
into  thorium  X,  and  the  latter,  in  turn,  is  changing  into  the 
emanation.  A  chemical  separation  is  necessary  to  effect  the 
removal  of  thorium  X  from  thorium,  because  both  types  of 
matter  are  non- volatile.  But  to  remove  the  emanation  no  such 
operation  is  needed,  because  in  this  case  the  matter  is  gaseous 
in  character,  and  separates  itself  from  the  matter  producing  it 
by  ordinary  gaseous  diffusion. 

Imparted  Radio-activity. — The  most  remarkable  property 
of  the  emanation  is  its  power  to  impart  temporary  radio- 
activity of  a  specific  kind  to  solid  bodies  with  which  it 
comes  into  contact  (Rutherford,  Phil.  Mag.,  1900,  V.,  49, 
p.  161). 

If  a  thorium  compound  is  kept  in  a  closed  vessel  for  several 
hours,  so  that  the  emanation  is  retained  and  prevented  from 
diffusing  away,  it  is  found  that  the  interior  of  the  vessel,  after 
removal  of  the  thorium,  is  itself  strongly  radio-active.  The 
character  of  the  rays  from  this  imparted  radio-activity  is 
distinct  from  that  of  the  thorium  itself,  being  more  penetrat- 
ing (compare  Fig.  20).  The  decay  of  this  imparted  radio- 
activity follows  the  usual  law,  decreasing  in  a  G.  P  with  the 
time,  and  reaches  half -value  after  11  hours.  A,  the  radio- 
active constant  for  this  case,  is,  therefore,  T7xlO"5.  All 
objects  become  radio  active  in  this  manner  independent  of 
their  nature,  and  the  imparted  radio-activity  is  of  the  same 
character.  Moreover,  if  the  surface  of  the  body  made  active 
is  scrubbed  with  sand  paper,  the  activity  is  to  a  large  extent 
removed  and  transferred  to  the  sandpaper.  Rutherford  found 
that  certain  acids  were  capable  of  dissolving  off  the  matter 
causing  the  radio  activity  from  a  platinum  wire  rendered  active 
by  the  thorium  emanation.  Hydrochloric  acid  and  sulphuric 
acid  especially  possessed  this  power,  whereas  nitric  acid  and 
alkalies  showed  very  little  effect.  The  activity  of  the  matter 
removed  is  in  no  case  destroyed.  Thus,  if  the  hydrochloric 
acid  used  be  evaporated  in  a  platinum  dish,  the  activity 
removed  from  the  wire  is  left  behind  on  the  dish  after  the 
evaporation  of  the  acid.  By  evaporating  aliquot  portions  of 


106  RADIO- ACTIVITY. 

the  acid  at  definite  intervals  of  time,  Rutherford  proved  that 
the  activity  of  the  matter  causing  the  phenomenon  decayed  at 
the  same  rate  when  dissolved  in  the  acid  as  it  does  when  kept 
on  the  wire  on  which  it  was  originally  deposited.  As  in  the 
other  cases,  the  matter  causing  these  phenomena  is  in  otherwise 
undetectable  quantity.  The  film  produced  is  quite  invisible 
and  unweighable. 

An  interesting  feature  experimentally  distinguishing  the 
type  of  active  matter  causing  the  imparted  radio-activity 
of  thorium  is  its  behaviour  in  an  electric  field.  If  a  central 
electrode  is  kept  negatively  charged  with  respect  to  the 
vessel,  the  imparted  activity  deposited  inside  the  vessel  from 
the  emanation  is  confined  solely  to  the  negative  electrode. 
The  matter  causing  the  imparted  activity  at  the  moment  of  its 
production  carries  a  positive  charge  and  travels  in  a  field  to 
the  negative  electrode.  In  this  way  it  is  possible  to  concen- 
trate the  whole  of  the  imparted  activity  on  a  fine  wire,  so  that 
the  latter  is,  weight  for  weight,  many  hundred  times  more 
active  than  the  original  thorium  compound.  Under  these 
circumstances  the  slightest  gain  in  weight  of  the  wire  could  be 
observed,  but  none  has  been  detected.  The  action  of  the  field 
does  not  in  any  way  affect  the  total  amount  of  radio-activity 
imparted  by  the  emanation,  but  merely  alters  the  distribution. 
For  a  fuller  consideration  of  this  phenomenon,  see  Rutherford, 
Phil  Mag.,  1903,  VI.,  5,  p.  95. 

The  interpretation  of  the  foregoing  facts  follows  without 
difficulty  on  the  explanation  adopted  in  the  other  cases. 
The  deposition  of  a  film  of  matter  by  the  emanation  on 
solids,  the  imparted  matter  again  possessing  a  specific  kind  of 
temporary  radio-activity,  obviously  points  to  a  third  radio- 
active change,  the  gaseous  emanation  changing  in  turn  into  a 
non-volatile  type  of  matter  which  settles  down  out  of  the  gas 
upon  whatever  provides  it  a  resting-place.  Being  positively 
charged  at  the  moment  of  production,  it  will  move  in  a  field 
and  be  directed  to  the  negative  electrode  exclusively.  The 
possession  of  this  charge,  no  doubt,  assists  its  deposition,  even 
when  no  field  is  acting,  by  causing  it  to  be  attracted  to 
uncharged  objects. 

Analysis  of  the  radio-activity  of  the  element  thorium  has 
thus  shown  that  it  is  made  up  of  at  least  four  parts,  due  to 


RADIO-ACTIVE  EMANATION  OF  THORIUM.         107 

four  specifically  distinct  types  of  matter — the  parent  thorium 
and  three  new  types  continuously  being  produced  from  it  by 
successive  change  : 

Thorium 

I 

Thorium  X 

I 

Emanation 

I 

Matter  causing  the 
imparted  radio-activity. 

It  must  be  admitted  that  a  series  of  changes  of  this  kind 
would  be  extremely  remarkable  if  they  were  considered  to  be 
due  to  ordinary  molecular  or  chemical  change.  The  absolute 
dissimilarity  of  the  successive  products  in  chemical  and 
physical  nature,  the  matter  in  question  passing  from  a  non- 
volatile state  to  the  gaseous  condition  and  again  back  into  the 
non  volatile  form,  is,  however,  what  is  to  be  expected  of  a 
sub-atomic  transformation.  If  radio-active  change  consisted 
in  the  successive  degradations  of  a  heavy  atom  into  lighter 
ones,  unless  it  happened  that  all  the  products  were  members 
of  the  same  family  of  elements  in  the  periodic  table,  it  is  to 
be  expected  that  the  successive  elements  resulting  would 
exhibit  the  widest  variation  in  their  nature.  The  fact  that 
some  families  of  elements  increase  in  volatility  with  increasing 
atomic  mass  (for  example,  the  series  lithium-caesium,  mag- 
nesium-mercury), while  others  (oxygen-tellurium,  fluorine- 
iodine)  follow  the  opposite  course,  together  with  the  existence 
of  very  light  elements  which  are  extremely  non-volatile — for 
example,  carbon  and  silicon — shows  that  nothing  can  be  pre- 
dicted of  the  physical  nature  of  the  products  resulting  from 
the  break  up  of  a  heavy  atom  unless  they  chance  to  belong  to 
the  same  family  in  the  periodic  table. 


CHAPTER   VIII. 


THE  THEORY  OF  ATOMIC  DISINTEGRATION. 

Radiations  remain  Unaltered  in  Character  during  Decay  and  Recovery. — 
Relation  between  Radiation  and  Continuous  Production  of  New 
Matter. — Both  Rays  and  New  Matter  Result  from  Parent  Element  by 
the  Same  Change.— Radio-activity  Measures  the  Number  of  Atoms 
Changing. — Non-separable  Activity.— Rate  of  Decay  a  Measure  of 
Rate  of  Change. — \N  Atoms  Change  per  Second  when  N  are  Present. 
— Mono-molecular  Types  of  Change. — Disintegration. — Conservation  of 
Radio-activity. — Ultimate  Products  of  Radio-active  Change.  -Predic- 
tion with  regard  to  Helium. — Cause  of  the  Initial  Irregularities  in 
the  Curves  of  Decay  and  Recovery  of  Thorium  A'  and  Thorium. — 
De-emanation. — Due  to  Alteration  of  Rate  of  Escape  of  Emanation. — 
Rate  of  Production  Constant  as  Theory  require*.— Disturbing  Effects 
of  Subsequent  Changes. —  General  Resume  of  the  Theory  of  Atomic 
Disintegration.— Latent  Energy  of  Atomic  Structure. — Explosive 
Character  of  the  Individual  Disintegrations. — Metabolons. — Infini- 
tesimal Quantity  of  the  Transition-forms. —  Unstable  Elements. — 
Criteria  of  Identification  — Average  Life. — Reason  of  the  Stability  of 
the  Elements. 

In  preceding  chapters  the  products  of  radio-active  change  have 
been  studied  by  their  radio-activity,  which  has  been  used  as  the 
means  by  which  infinitesimal  quantities  of  matter  have  been 
brought  within  range  of  experimental  investigation.  No  enquiry 
has,  so  far,  been  pursued  as  to  the  nature  of  radio-activity  except 
that  it  has  been  shown  to  be  for  the  most  part  maintained  by  the 
continuous  production  of  fresh  active  matter.  It  remains  to 
frame  a  consistent  physical  explanation  of  the  nature  of  radio- 
activity itself  (Rutherford  and  Soddy,  Phil.  Mag.,  1903,  YL, 
5,  p.  576).  The  discussion  in  Chapter  \.  of  the  nature  of 
the  a  and  ft  radiations  assists  us  materially  in  this  task. 
Evidence  was  then  brought  forward  to  show  that  the  a-rays 


110  RADIO-ACTIVITY. 

are  caused  by  the  radial  expulsion  from  the  active  substance 
of  positively-  charged  particles  of  atomic  dimensions  (Ruther- 
ford), while  the  /3-rays  are  high-velocity  cathode  rays,  or 
negatively-charged  particles  of  sub-atomic  dimensions  (J.  J. 
Thomson  and  Becquerel).  Now,  in  the  changes  suffered  by 
radio-activity  during  the  processes  of  decay  and  recovery  that 
have  been  considered  the  character  of  the  radiation  is  not 
sensibly  affected.  Hence  we  must  regard  these  changes  in 
radio-activity  as  being  caused  by  alterations  in  the  number 
of  ^articles  projected  as  raya  in  nnjfr  fimp.  This  considera- 


tion is  important,  for  it  precludes  the  view  that  the  decay 
of  the  activity  of  thorium  X,  for  example,  is  analogous  to 
the  decay  of  the  radiations  from  a  hot  body  cooling.  The 
character  of  the  particles  projected  remains  unaltered,  and 
the  weakening  of  the  activity  is  caused  by  a  diminution  in  the 
number  emitted  in  unit  time.  Similarly,  the  only  distinction 
between  the  radio-activity  of  a  powerfully  active  element  like 
radium  and  a  feebly  active  element  like  thorium  is  that  a 
greater  number  of  radiant  particles  are  expelled  per  unit- 
weight  in  unit  time.  Considered  from  this  point  of  view  the 
emanation  of  thorium,  thorium  X,  &c.,  are  far  more  power- 
fully active  substances  than  radium.  We  have  seen  that,  in 
the  case  of  thorium,  thorium  X,  the  emanation,  and  the  matter 
causing  the  imparted  activity,  each  type  of  matter  not  only 
expels  rays,  but  produces  the  next  new  type  of  matter  also.  So 
radio-activity  will  be  completely  defined  as  the  simultaneous 
occurrence  of  two  processes:  (1)  The  expulsion  of  charged 
particles  with  enormous  velocity  ;  (2)  the  production  of  new 
types  of  matter  in  minute  amount,  which  can  be  investigated 
when  they  also  are  radio-active.  What  is  the  precise  connection 
between  these  two  processes  ?  Thorium  X  furnishes  a  convenient 
example,  for  (1)  the  expulsion  of  rays  can  be  determined  by 
measuring  its  radio-activity,  (2)  the  production  of  new  matter, 
by  measuring  its  emanating  power.  It  was  found  (Rutherford 
and  Soddy,  Phil.  Mag.,  1902,  VI,  4,  p.  579)  that,  when  one  part 
of  thorium  X  is  kept  in  solution  and  its  emanating  power  tested 
from  day  to  day,  it  decayed  with  time  according  to  the  same 
law  and  at  the  same  rate  as  the  radiations  from  another  part  of 
the  same  sample  that  had  been  evaporated  to  dry  ness  in  a  plati- 
num dish.  This  points  to  the  conclusion  that  the  two  processes 


THEORY  OF  ATOMIC  DISINTEGRATION. 


Ill 


(1)  and  (2)  are  causally  connected.  Another  example,  giving 
the  same  result,  is  to  be  found  in  the  first  paper  of  Kutherford 
(Phil  Mag.,  1900,  V.,  49,  p.  170)  on  the  imparted  activity  of 
thorium.  Here  we  have  (1)  the  radiation  from  the  radio-active 
emanation,  (2)  the  production  of  the  new  matter  which  causes 
the  imparted  activity.  A  steady  current  of  air  was  blown  over 
a  thorium  compound  into  the  tube  shown  in  Fig.  29  in  the 
direction  of  the  arrows  for  several  hours  continuously.  At  the 
commencement  the  ionisation  current  due  to  the  rays  from  the 
emanation  was  measured  by  connecting  the  four  electrodes 
A,  B,  C,  D  separately  with  the  electrometer,  the  exterior  of 
the  tube  being  connected  with  the  -f  pole  of  a  battery  giving 


Earth 


FIG.  29. 


300  volts,  the  other  pole  of  which  was  earthed.  The  decay  of 
activity  suffered  by  the  emanation  in  the  time  required  for  its 
passage  through  the  tube  caused  the  current  to  the  electrode 
A  to  be  greater  than  to  B,  and  the  ratio  of  the  currents  due  to 
the  emanation  at  the  four  electrodes  was  determined. 

With  the  above  arrangement  the  whole  of  the  imparted 
activity  produced  by  the  thorium  emanation  is  deposited  on  the 
electrodes,  which  are  negatively  charged  with  respect  to  the 
tube.  After  the  emanation  had  been  kept  passing  for  several 
hours  the  electrodes  were  removed  from  the  apparatus  and 
the  imparted  radio-activity  on  each  determined.  It  was 
found  that  the  four  electrodes  exhibited  substantially  the 
same  ratio  in  their  imparted  activities  as  in  the  determination 


112  RADIO-ACTIVITY. 

of  the  current  due  to  the  emanation.  That  is  to  say,  as  the 
radio-activity  of  the  emanation  decays  with  time,  its  power 
to  impart  radio-activity  to  its  surroundings  correspondingly 
diminishes.  The  number  of  particles  projected  as  rays  from 
the  emanation  during  its  change  into  the  new  matter  cau  sing 
the  imparted  activity  is  proportional  to  the  amount  of  the 
latter  (as  measured  by  its  radio-activity)  produced. 

It  follows,  therefore,  that  both  the  projected  particles,  or 
rays,  and  the  new  matter  are  products  of  the  parent  type  of 
matter  by  the  same  change.  The  rays  constituting  the  radio- 
activity of  thorium  are  expelled  at  the  moment  the  atom  of 
thorium  changes  into  thorium  X,  the  rays  constituting  the  radio- 
activity of  thorium  X  are  expelled  at  the  moment  the 
thorium  X  atom  changes  into  the  emanation,  and  the  rays 
from  the  emanation  atom  are  expelled  from  it  at  the  moment 
it  changes  into  the  again  non-volatile  type  of  matter  causing 
the  imparted  activity.  The  rays  from  the  latter  are  derived 
from  still  further  changes.  The  atomic  theory,  pnt  .briefly ^  j^ 
that_  the  atoms  of  any  one  clement  arc  all  alike,  and  different 
from  those  of  any  other.  Hence,  wejna^  assume  thaLtke-same 

Whether  one  or  more  is  expelled  will  be  considered  in  Chapter 
XI.,  but  it  is  immaterial  to  the  present  argument.  The  number 
must  always j»e  the  same  for  the  same  kind  of  atom  iinrfergoim^ 
Jhe_samp.  r.ha,ngft  Thus  the  activity  of  any  single  type  of 
radio-active  matter,  in  the  sense  of  the  number  of  particles 
expelled  from  it  in  unit  time,  furnishes  a  measure  of  the 
number  of  atoms  undergoing  change. 

Several  important  and  definite  conclusions  follow  from 
this  point  of  view,  which  have  in  many  cases  been  found 
capable  of  direct  experimental  test.  In  the  first  place,  since 
the  production  of  thorium  X  from  thorium  proceeds  at  a  con- 
stant rate  in  any  given  mass  of  thorium  compound  without 
reference  to  the  physical  and  chemical  conditions  (see  p.  93), 
it  follows  that  a  definite  fraction  of  the  total  number  of  thorium 
atoms  are  changing  in  each  unit  of  time  into  thorium  X,  and 
this  change  is  accompanied  by  the  projection  into  space  of  a 
definite  number  of  a  particles.  Hence,  to  whatever  chemical 
process  thorium  is  subjected,  it  should  prove  impossible  to 
entirely  free  it  from  radio-activity — that  is  to  say,  there  must 


THEORY  OF  ATOMIC  DISINTEGRATION. 

exist  a  certain  constant  non-separable  radio-activity.  Now 
this  has  been  shown  to  be  true,  so  far  as  our  experimental 
knowledge  goes,  not  only  of  thorium,  but  al  so  of  uranium  and 
radium,  and  in  each  case  it  is  important  to  note  that  the 
non-separable  activity  consists  entirely  of  a-rays.  The__next 
dftdiu».t.inn  from  the  position  that  the  expulsion  of  t.hft  rays 
accompanies  the  change  of  the  atom  is  that  radio-active 
matter  must  be  continuously  diminishing  in  quantity.  A  radio- 
active atom  is  a  changing  atouL  For  feeble  examples,  like 
uranium  and  thorium,  it  is  not  necessary  that  the  change 
should  be  great  enough  to  be  appreciable.  It  is  probable  that 
in  these  cases  the  quantity  in  existence  remains  practically  con- 
stant over  very  long  epochs  of  time.  But  if  this  is  true,  it  follo_ws 
equally  that^the  actual  quantity  of  uranium  X  and  thorium  X 
produced  from  them  must  also  be  excessively  small.  In  order 
that~Th~is~  insignificant  amount  of  matter  should  be  able  to  give 
sufficient  rays  to  be  appreciated  it  must  be  changing  much 
faster  than  the  original  element,  and  its  "quantity  in  conse- 
quence must  rapidly  diminish.  Hence  its  radio-activity 
decays,  and  the  rate  of  decay,  together  with  the  intensity  of 
the  radio-activity  of  unit  quantity,  depend  upon  whether  it  is 
changing  slowly~br~rapidlyir~ 

The  radio-activity  of  an  intensely  active  substance  must 
quickly  decay.  On  the  general  assumption  that  the  order  of  the 
total  quantity  of  energy  liberated  in  the  form  of  rays  may  be 
expected  to  be  similar  for  each  type  of  radio-active  atom, 
the  argument  may  be  inverted.  The  more  rapidly  the 
activity  decays  the  more  intensely  radio-active  (considered 
with  reference  to  unit-weight)  must  the  type  of  matter  be. 
Marckwald's  work  on  polonium  (Chapter  II.)  is  an  example  of 
this  principle.  We  have  seen  that  the  activities  of  uranium  X, 
thorium  X,  the  emanation  of  thorium,  and  the  matter  imparted 
by  the  latter  to  other  objects,  all  decay  in  a  G.P.  with  the 
time  at  characteristic  rates,  represented  by  the  equation 
(p.  92) 


in  which  A  is  a  numerical  constant  different  for  each  type  of 

matter.    The  expression  -=?  represents  the  ratio  of  the  number 
ID 


114  RADIO-ACTIVITY. 

of  rays  expelled  in  unit  time  for  any  time  t  to  the  number  ex- 

pelled initially,  and  may  be  replaced  by  -  ,  where  nt  represents 

n0 

the  number  of  atoms  of  the  substance  changing  in  unit  time, 
for  time  t,  and  n0  the  number  initially,  so  that 

"-=«-«. 

UQ 

To  find  the  rate  of  change  of  the  original  system,  let  N0  =  the 
total  number  of  atoms  originally  present  and  Nt  the  number 
.  eft  unchanged  at  time  /, 


0  =  N«  when  t  =  0, 


- 

HT 

(IN 

Differentiating,         -=-  —  -  AN  . 
(MI 

The  result  is  thus  a  very  simple  one.  On  the  view  that  the 
number  of  rays  expelled  is  a  measure  of  the  number  of  atoms 
of  the  system  undergoing  change,  the  geometrical  decrease  of 
the  radio-activity  is  due  to  the  geometrical  decrease  of  the 
number  of  atoms  of  the  changing  system.  The  rate  of  change 
of  the  atoms  undergoing  radio-active  change  is  always  propor- 
tional to  the  amount  remaining  unchanged.  If  N  atoms  of 
any  type  of  radio-active  matter  are  present,  AN  change  per 
second,  where  A  is  the  radio-active  constant.  From  the  values 
of  A  already  given  it  follows  that  about  ¥xTtn  of  the  thorium 
emanation,  g-Q-^o^^th  of  thorium  X,  and  -3  o~o  jj^^th  °f  uranium 
X  undergoes  change  per  second. 

Now,  the  law  of  radio-active  change  just  developed  is  already 
well  known  to  us  as  the  law  followed  by  a  particular  kind  of 
chemical  reaction  —  viz.,  that  known  as  mono-molecular.  In 
reactions  which  are  of  the  type  of  the  decomposition  of  the 
single  molecule  into  simpler  parts  the  quantity  of  the  sub- 
stance undergoing  change  under  constant  conditions  decreases 
in  a  geometrical  progression  with  the  time,  owing  to  the 


THEORY  OF  ATOMIC  DISINTEGRATION.  115 

amount  changing  in  unit-time  being  a  constant  fraction  of  the 
amount  present.  Both  are  simple  consequences  of.  the  law 
of  probabilities,  the  number  of  changes  occurring  under 
•constant  conditions  increasing  with  the  number  of  changing 
systems.  But  the  case  is  completely  different  when  the 
reaction  is  not  of  the  type  of  a  decomposition  but  of  a  com- 
bination. For  here,  in  order  for  the  change  to  occur,  the 
combining  molecules  or  atoms  must  first  approach  within  each 
other's  spheres  of  influence.  The  rate  of  change,  in  conse- 
quence, depends  upon  the  concentration,  and  this  diminishes 
with  the  progress  of  the  change.  The  rate  of  change  proceeds 
in  these  cases  according  to  some  power  of  the  number  of 
changing  systems  present.  Hence,  radio-active  change  cannot 
be  of  the  nature  of  a  combination  together,  or,  as  a  chemist 
would  say,  the  polymerisation,  of  the  atoms  of  the  active 
element,  but  must  be  due  to  their  decomposition  or  •  disin- 
tegration. The  term  changing  atom  can  now  be  logically 
replaced  by  the  more  definite  conception  expressed  by  the 
use  of  the  term  disintegrating  atom,  with  considerable  advan- 
tage to  the  clearness  of  the  mental  picture  conveyed. 

The  Conservation  of  fiadio-activity. — The  simple  law  of  radio- 
active change — that  in  N  atoms  of  any  type  of  radio-active 
matter  AN  disintegrate  per  second,  where  A  is  a  constant  charac- 
teristic for  the  type  of  matter  considered  and  unalterable  by 
any  known  agency — leads  to  some  general  deductions  of  a  far- 
reaching  character.  Kadio-activity  is  derived  either  (1)  from 
the  slow  disintegration  of  some  "  permanent "  radio-element — 
i.e.,  an  element  changing  excessively  slowly,  so  that  the  amount 
does  not  perceptibly  alter  from  year  to  year ;  or  (2)  the  more 
rapid  disintegration  of  transition-forms  of  matter  which  are 
maintained  in  existence  because  they  are  continually  being 
reproduced  by  (1).  The  radio-activity  due  to  (1)  must  be 
-constant  over  ordinary  periods  of  time.  With  regard  to  (2),  a 
little  consideration  shows  that  the  quantity  of  each  transition- 
form  in  existence  is  also  constant,  and  must  bear  some  fixed 
relation  to  the  quantity  of  the  parent  element,  for  it  is  that 
quantity  at  which  so  much  has  accumulated  that  the  quantity 
breaking  up  per  second  exactly  balances  the  constant  quantity 
produced  per  second.  Therefore,  each  transition-form  contributes 
a  fixed  proportion  of  the  radio-activity,  so  that  the  sum  total  of 

I  2 


116  EADIO- ACTIVITY. 

radio-activity  is  a  constant.  All  that  is  effected  by  the  chemical 
separations  that  have  been  discussed  is  to  remove  one  or  more 
of  the  transition  forms.  But  the  sum  of  the  activities  is 
still  the  same  as  before  separation.  Hence,  if  the  radio-activity 
of  a  preparation  increases  with  lapse  of  time,  this  of  itself  is 
evidence  that  during  its  preparation  a  transition-form  has 
been  separated  from  it,  and  if  looked  for  it  will  be  found  that 
the  activity  of  the  transition-form  steadily  decays  concomit- 
antly  with  the  steady  rise  of  activity  of  the  preparation. 
This  is  the  principle  of  the  conservation  of  radio-activity,  and 
holds  true  under  all  conditions  that  have  yet  been  investi- 
gated, If  it  were  otherwise,  the  value  of  A  would  suffer 
alteration,  and  this  is  contrary  to  experience,  so  far  as  our 
present  experimental  knowledge  is  concerned.  After  very 
long  periods,  probably  comparable  with  the  age  of  the  earth, 
the  radio-activity  of  any  given  quantity  of  one  of  the  parent 
radio-elements,  uranium  or  thorium  for  example,  must  diminish, 
it  is  true,  unless  there  is  some  upward  process  of  evolution  not 
yet  recognised  which  is  re-forming  the  heavy  elements  out  of 
the  lighter  ones,  and  so  maintaining  their  quantity.  But  the 
principle  of  conservation  applies  rigidly  only  when  some  fixed 
amount  of  radio-element  is  considered,  and  this  is  spontane- 
ously growing  less.  The  proportionate  change  in  these  cases  is 
so  small  that  it  may  be  neglected  for  ordinary  periods  of  time. 
Ultimate  Products. — The  question  arises,  since  all  radio-active 
matter  must  be  changing,  What  are  the  final  products  result- 
ing from  the  disintegration  of  the  radio-elements  when  they 
at  length  assume  a  stable — i.e.,  a  non- radio-active — form  1 
Obviously  no  evidence  can  be  obtained  in  this  direction  by  direct 
experiment,  at  least  for  the  feebly-active  elements  uranium 
and  thorium,  except  by  operating  with  very  large  quantities 
over  long  periods.  This  has,  fortunately,  been  done  for  us  by 
Nature  in  the  minerals  containing  these  elements,  for  we  must 
suppose,  having  no  evidence  to  the  contrary,  that  the  changes 
have  been  proceeding  continuously  over  geological  epochs  in 
these  elements  as  they  occur  in  the  crust  of  the  earth,  and 
that,  therefore,  the  ultimate  products  of  the  changes  tend  to 
accumulate  in  the  radio-active  minerals  in  quantities  sufficient 
to  be  detected  by  ordinary  chemical  means.  Under  favour- 
able circumstances  they  should  appear  in  the  minerals  as 


THEORY  OF  ATOMIC  DISINTEGRATION.  117 

invariable  companions  of  the  radio-elements.  An  examina- 
tion of  the  minerals  containing  uranium  and  thorium,  many 
of  which,  as  we  now  know,  contain  radium  also,  shows 
that  there  are  usually  present  a  very  large  number  of 
elements  in  larger  or  smaller  quantities.  The  frequent 
presence  of  one — viz.,  helium — is,  however,  very  remark- 
able. When  Sir  William  Ramsay  discovered  this  element  for 
the  first  time  in  terrestrial  sources,  he  drew  attention  to  the 
curious  fact  that  it  only  occurs  in  those  minerals  which  contain 
uranium  or  thorium — i.e.,  as  we  know  now,  are  radio-active. 
On  this  account  the  case  of  helium  is  very  suggestive.  For 
the  element  belongs  to  the  family  of  inert  gases  which  are 
incapable  of  entering  into  chemical  combination,  and,  once 
separated  from  the  mineral,  by  heat  or  solution,  cannot  be 
made  to  re-combine  with  it  again.  If,  however,  helium  were 
liberated  by  radio-active  change,  through  the  mass  of 
the  substance,  it  is  possible  that  it  might  be  mechanically 
prevented  from  escaping.  Rutherford  and  Soddy  discovered 
(Phil.  Mag.,  1903,  VI.,  5,  p.  453)  that  this  is  the  case 
with  the  gaseous  emanations  of  thorium  and  radium  which, 
under  certain  circumstances,  remain  stored  up  in  the  compound 
producing  them,  and  do  not  escape.  They  suggested  that 
helium  might  be  a  disintegration  product  of  one  of  the  radio- 
elements.  Thanks  to  the  great  advances  made  of  recent  years 
in  gas  manipulation  by  Sir  William  Ramsay  and  his  colleagues 
in  their  work  on  the  rare  gases  of  the  atmosphere,  which 
necessitated  the  handling  of  vamshingly  small  quantities  of 
gases,  this  prediction  has  been  verified  experimentally  in  the 
case  of  radium  (p.  157). 

We  are  now  in  a  position  to  study  somewhat  more  exactly 
than  was  before  possible  the  nature  of  the  radio-active  changes 
occurring  in  thorium.  Owing  to  its  complex  nature,  the 
parent  element  producing  in  turn  several  products,  each  of 
which  is  radio-active,  the  simple  laws  arrived  at  by  consider- 
ing each  change  by  itself,  without  reference  to  the  effect  of 
the  subsequent  changes,  are  not  often  strictly  applicable 
without  modification.  Thus,  the  experimental  curves  obtained 
(Fig.  30,  reproduced  from  Fig.  25)  in  the  determinations 
of  the  rate  of  decay  of  the  activity  of  thorium  X,  and 
the  rate  of  recovery  of  the  activity  of  thorium,  exhibit 


118 


RADIO- A  CTIVIT  Y. 


for  the  first  two  days  divergencies  from  the  normal  course 
afterwards  followed.  These  are  at  once  capable  of  explana- 
tion if  the  other  products  of  the  change  besides  thorium  X 
—viz.,  the  emanation  and  the  matter  causing  the  imparted 
activity— are  taken  into  account.  Before  the  separation 
of  thorium  X,  the  activity  of  a  thorium  compound  is  due 
to  the  several  activities  of  (1)  thorium,  (2)  thorium  X, 
(3)  emanation,  (4)  matter  causing  imparted  activity.  By 
the  chemical  operation  (2)  is  separated,  leaving  (1),  (3),  (4), 


10       12       14      16       18 
Time  in  Days. 

FIG.  30. 


20     22 


The  emanation  (3),  being  very  short-lived,  loses  its  activity 
within  a  few  minutes  after  the  separation,  and  need  not  be 
practically  considered.  But  (4),  the  matter  causing  imparted 
activity,  requires  1 1  hours  for  the  activity  to  fall  to  half  value. 
Hence  the  recovery  of  activity  of  the  thorium,  due  to  the 
regeneration  of  thorium  X,  will  be  offset  by  the  simultaneous 
decay  of  the  activity  of  the  matter  causing  the  imparted 
activity.  Thus  the  activity  of  the  thorium  compound  does 
not  commence  to  recover  at  once,  but  at  first  diminishes 


THEORY  OF  ATOMIC  DISINTEGRATION. 

owing  to  the  decay  of  the  imparted  activity  present.  The 
truth  of  this  conclusion  was  tested  by  separating  thorium  X 
a  great  many  times  at  short  intervals  from  the  thorium,  so  as 
to  give  the  imparted  activity  produced  by  the  thorium  X, 
originally  present  before  the  first  separation,  time  to  com- 
pletely decay.  Under  these  circumstances,  it  is  found  that 
the  recovery  curve  commences  to  rise  immediately  in  the 
normal  manner  from  a  25  per  cent,  minimum,  and  the 
initial  irregularity  is  completely  absent.  The  same  considera- 
tions hold  good  for  the  decay  curve  of  thorium  X.  At 
first  the  imparted  activity  produced  in  the  further  change  of 
thorium  X  causes  the  activity  of  the  latter  to  increase,  until 
the  point  is  reached  in  about  two  days  at  which  the  amount 
of  imparted  activity  produced  balances  its  decay  with  time. 
Then  the  augmentation  of  the  activity  stops,  and  the  steady 
decay  of  the  activity  of  thorium  X,  to  half-value  in  four 
days,  proceeds  after  the  second  day  according  to  the  simple 
geometrical  progression. 

De-emanation. — Another  point  which  will  repay  discussion 
before  leaving  the  element  thorium,  is  the  great  variation  in  the 
emanating  powers  of  different  compounds,  and  of  the  same  com- 
pound under  different  conditions.  For  the  law  that  AN  atoms 
change  when  N  atoms  are  present  necessitates  equally,  with  a 
definite  and  unalterable  radio-activity,  a  correspondingly 
definite  rate  of  prouuction  of  the  emanation.  Now,  although 
the  radio-activity  of  thorium  compounds  is  proportional  to  the 
quantity  present,  and  is  not  affected  by  chemical  or  physical 
means,  it  is  found  that  the  emanating  power  of  a  compound 
often  bears  little  relation  to  the  amount  of  thorium  con- 
tained in  it.  Certain  solid  compounds,  like  the  oxide,  but 
more  particularly  the  hydroxide  and  carbonate,  give  off  a 
large  amount  of  emanation,  while  others,  like  the  nitrate, 
oxalate,  &c.,  give  off  very  little  indeed.  It  was  found  however, 
that,  when  dissolved  in  aqueous  solutions,  all  compounds  of 
thorium  are  equally  effective  in  this  respect,  and  the  emanat- 
ing power  may  be  taken  to  be  at  its  maximum  in  solution. 
Rutherford  noticed,  in  his  original  observations  of  the  emanat- 
ing power  of  thorium,  that  if  the  oxide  is  intensely  heated 
the  emanating  power  is  permanently  reduced  to  a  few  per 
cent,  ©f  its  original  value,  and  preparations  so  treated  are 


120  RADIO-ACTIVITY. 

frequently  referred  to  as  "  de-emanated."  But  if  the  de- 
emanated  oxide  is  subjected  to  a  series  of  chemical  processes, 
and  again  converted  into  a  soluble  compound,  it  is  found  that 
the  normal  maximum  emanating  power  is  possessed  by  the 
solution,  showing  that  the  phenomenon  of  de-emanation  is  to 
be  associated  with  a  particular  physical  state.  The  emanating 
power  of  a  given  thorium  preparation  is  much  affected  by 
temperature,  being  very  small  at  temperatures  of  about— 100°0., 
and  being  increased  by  rising  temperature  up  to  the  point — 
about  a  red-heat — at  which  the  process  of  de-emanation  sets 
in  The  power  of  a  thorium  compound  to  impart  activity  to  its 
surroundings  varies  correspondingly  with  its  emanating  power. 
All  these  variations  are  satisfactorily  accounted  for  by  the 
view  that  the  rate  of  production  of  the  emanation  is,  as  theory 
requires,  constant  and  proportional  to  the  quantity  of  thorium 
(or  more  strictly  of  thorium  X)  present,  but  that  the  rate 
of  escape  of  the  gaseous  emanation  from  the  compound 
producing  it  is  much  affected  by  temperature,  solution,  and  the 
chemical  and  physical  state  of  the  compound.  De-emanation, 
for  example,  is  brought  about  by  an  alteration  in  the  physical 
state  of  the  oxide,  which  retards  the  escape  into  the  surround- 
ing atmosphere  of  the  emanation,  which  is  formed  uniformly 
through  the  mass  of  the  compound.  Owing  to  the  rapid 
decay  of  the  activity  of  the  thorium  emanation,  a  relatively 
slight  decrease  in  its  rate  of  escape  will  greatly  affect  the 
value  of  the  observed  emanating  power.  In  consequence, 
the  radio-activity  of  the  emanation  appears  as  a  part  of  that 
of  the  solid,  and  the  imparted  activity  it  produces  is  retained 
in  the  compound  instead  of  being  dissipated  among  surround- 
ing bodies.  Hence  the  radio-activity  of  a  highly  emanat- 
ing compound  like  thorium  hydroxide  should  be  increased 
by  strong  ignition,  and  the  increase  should  reach  a 
maximum  about  two  days  after  the  process,  when  the 
imparted  activity  produced  by  the  imprisoned  emanation 
obtains  its  equilibrium-value.  This  was  found  to  be  the  case. 
The  activity  of  a  sample  of  thorium  hydroxide  rises  slowly 
during  the  first  two  days  after  ignition  to  a  maximum  about 
120  per  cent,  of  the  former  value.  The  converse  experiment 
is  to  take  a  non-emanating  compound  and  suddenly  convert  it 
into  a  highly-emanating  one.  This  was  accomplished  by 


THEORY  OF  ATOMIC  DISINTEGRATION.  121 

keeping  the  highly-emanating  hydroxide  in  liquid  air  for 
some  days,  so  that  the  emanation  is  condensed  and  does  not 
escape,  and  its  radio-activity  and  the  imparted  activity  it  pro- 
duces is  added  on  to  the  activity  of  the  compound.  On 
removing  it  from  the  liquid  air  and  allowing  it  to  warm  up  the 
activity  slowly  decays  about  20  per  cent,  during  the  first  few 
days,  owing  to  the  emanation  now  being  free  to  escape  from 
the  system  and  to  carry  away  the  imparted  activity  to 
surrounding  objects. 

No  simple  general  treatment  is  here  possible  of  the  disturbing 
effects  of  subsequent  changes.  The  following  cases  may  be 
qualitatively  recognised.  If  a  body,  A,  changes  into  B,  and 
B  changes  into  C  more  rapidly  than  A  into  B,  the  decay  curve 
of  A  will  show  a  preliminary  increase  in  activity,  and  then  a 
decrease  to  zero  at  the  rate  of  change  of  A  into  B.  If,  how- 
ever, B  changes  into  C  at  a  slower  rate  than  A  into  B,  the 
decay  curve  will  exhibit  a  normal  course  down  to  a  certain 
point,  and,  if  this  point  is  calculated  as  zero,  the  rate  of  change 
may  be  considered  without  error  as  the  true  rate  of  change  of 
A  into  B.  From  this  point  the  activity  decays  to  zero  at  a 
rate  slower  than  before — viz.,  that  of  B  into  C.  These  cases 
are  complicated  experimentally  when  one  of  the  stages  consists 
of  a  gaseous  type  of  matter,  like  the  emanation,  but  each  case 
•can  be  worked  out  for  itself  from  first  principles.  Further 
very  instructive  illustrations  will  be  considered  in  the  case  of 
radium.  It  has  been  assumed  in  the  above  cases  that  a-rays 
are  expelled  at  each  change. 

In  the  above  consideration  of  the  nature  of  radio-active 
change,  for  the  sake  of  simplicity  and  to  avoid  confusion,  the 
case  of  thorium  has  been  taken,  this  being  the  element  for 
which  the  theory  was  first  worked  out.  But  it  applies  equally 
to  every  case,  so  far  as  is  at  present  known,  and  the  treatment 
of  the  remaining  cases  is  much  simplified  by  the  consistent 
application  of  the  theory. 

It  will  be  convenient  to  conclude  the  chapter  by  a  summary 
in  general  terms  of  the  nature  of  the  processes  that  the  study 
of  the  radio-elements  has  revealed. 

Instead  of  regarding  each  atom  as  a  constant  source  of  energy, 
in  the  form  of  rays,  the  disintegration  theory  regards  the 
property  as  due  to  a  fixed  proportion  of  the  total  number 


122  RADIO-ACTIVITY. 

of  atoms  which  are  undergoing  disintegration.  During  the 
instant  of  their  disruption  they  fling  away  fragments  of 
themselves  into  space  in  the  form  of  radiant  particles.  Kadio- 
activity,  being  thus  a  property  of  a  fixed  proportion  of  the 
active  matter  in  each  instant,  behaves  in  many  ways  as  an 
atomic  property  contributed  equally  by  all  the  atoms.  The 
vast  majority  are  for  the  time  being  ordinary  inactive  atoms 
with  a  specific  and  characteristic  material  nature  which  is  not 
affected  by,  or  does  not  exert  any  influence  on,  the  character 
of  the  additional  set  of  properties  contributed  by  the  fraction 
disintegrating.  The  dual  character  of  the  properties  of  the 
radio-elements  is  thus  simply  explained. 

The  disruption  of  the  individual  atom  is  a  sudden 
and  explosive  process,  but  differs  from  that  of  ordinary 
explosion  in  the  fact  that  the  explosion  of  an  atom  exerts 
no  influence  on  the  rate  of  explosion  of  its  neighbours. 
In  an  explosive  substance — as,  for  example,  fulminate  of 
mercury  or  acetylene — the  disturbance  occasioned  by  the 
disintegration  of  a  single  molecule  is  the  cause  of  the  explosion 
of  the  surrounding  molecules,  so  that  the  whole  mass  deto- 
nates in  a  very  short  interval  of  time.  In  the  case  of  the 
disintegrating  atoms  the  cause  of  the  disintegration  is  at 
present  unknown.  It  proceeds  at  a  definite  rate,  a  fixed 
fraction  of  the  total  atoms  disintegrating  in  the  unit  of 
time,  without  hindrance  or  acceleration  by  any  agency  known. 
Radio-activity  and  the  processes  that  give  rise  to  it  appear 
to  lie  totally  outside  the  sphere  of  known  molecular  forces. 
No  change  in  the  chemical  and  physical  properties  of  the 
atom  appears  to  occur  as  it  approaches  the  termination  of 
its  existence  as  a  separate  entity.  Suddenly,  and  without 
previous  indication,  it  flies  to  pieces  by  some  internal 
cataclysm,  the  cause  of  which  we  can  only  conjecture.  The 
vast  store  of  energy  bound  up  with  its  internal  structure, 
and  which  make  it  a  stable  system  to  all  the  relatively  insig- 
nificant forms  of  energy  manifested  in  an  ordinary  chemical 
and  physical  change,  are  revealed  when  the  structure  flies 
to  pieces.  The  internal  energy  of  the  chemical  atom  becomes 
for  the  first  time  knowable  when  it  disintegrates.  Because 
the  atom  has  never  in  any  other  process  been  observed  to 
change,  and  because  it  does  change  in  exhibiting  radio- 


THEORY  OF  ATOMIC  DISINTEGRATION.  123 

activity,  we  have  the  explanation  of  the  fact  that  the  energy 
associated  with  matter  in  the  rays  of  the  radio-elements  is 
of  a  higher  order  of  magnitude  than  has  ever  been  dealt  with 
in  science  before.  Eadio-activity  thus  draws  its  supply  of 
energy  from  a  previously  untapped  reservoir — the  latent 
energy  associated  with  the  atomic  structure. 

Atomic  disintegration  would  have  been  a  difficult  process 
to   establish  by  direct   experiment   as   the   cause   of    radio- 
activity had  the  phenomenon  not  been  somewhat  complex. 
The   complexity  of   radio-activity,  especially  in   the  case  of 
the   element  thorium,  made  it  possible  that  so  far-reaching 
a  conception  as  the  one  put  forward  should  be  subjected  to 
rigorous  experimental  verification.     For  if  the  disintegrating 
atom  passed  from  its  initial  to  its  final  state  in  one  change, 
as  in  the  analogous  molecular  cases  of  explosion,  it  would 
have  been  difficult  to  have  obtained  much  knowledge  of  the 
process.    Fortunately,  this  is  not  the  case.    The  thorium  atom 
only  passes  to  its  final  state  after  at  least  five  separate  and 
successive  disintegrations,  each  of  which,  considered  by  itself, 
is  suddenly  and  explosively  consummated,  so  far  as  the  indi- 
vidual atom  is  concerned,  by  the  radial  expulsion  of  fragments 
in  the   form   of   a   particles.     In  consequence,  there  exist  a 
certain  number  of  short-lived  transition-forms  of  matter  inter- 
mediate between  the  initial  and  the  final  atoms  which  result 
from  the  process.     The  term  "  Metabolon  "  has  been  proposed 
by  Rutherford  and  Soddy  to  denote  unstable  atoms  of  this 
character,  and  the  name  indicates  change,  the  essential  feature 
which   characterises   them,   and   the  reason  why  they  come 
within  the  range  of  our  methods  of  investigation.     A  meta- 
bolon  is  an  atom  with  a  limited  life.     While  it  exists  it  is 
a  normal  atom,  possessing  the  ordinary  attributes  of  matter. 
At   the   moment    of    its    disruptive   change   it   exhibits   the 
property  of  radio-activity. 

Situated  as  we  are,  the  observers  for  a  short  time  only 
of  a  process  of  evolution  which  has  been  going  on  for 
indefinite  ages,  our  knowledge  of  the  unstable  atoms  is 
necessarily  limited  by  certain  definite  considerations.  In 
the  first  place,  since  the  process  is  not  affected  by  any 
known  agency,  we  must  assume  that  it  has  proceeded  con- 
tinuously during  past  ages  at  the  same  rate,  for  any  given 


124  RADIO-ACTIVITY. 

type  of  matter,  as  it  is  proceeding  at  present.  Hence  it 
follows  that  the  initial  process  of  disintegration  must  be 
excessively  slow,  in  order  that  some  of  the  matter  disin- 
tegrating should  survive  to  the  present  day.  The  initial 
type  of  matter  will  be  termed  the  parent-element.  For  any 
ordinary  quantity  of  parent- element,  considered  over  any 
ordinary  period  of  time,  the  actual  quantity  of  the  transition- 
forms  of  matter  resulting  from  the  disintegration  must  be 
excessively  minute.  The  quantity  of  the  transition -form  pro- 
duced, moreover,  cannot  accumulate  beyond  a  certain  limiting 
or  equilibrium  value,  which  is  attained  when  the  amount 
changing  per  second  equals  the  amount  produced  per  second, 
which  is  the  condition  of  radio-active  equilibrium.  Since  the 
rate  of  production  is  slow,  and  the  rate  of  change  of  the 
transition-form  is  rapid,  the  equilibrium  quantity  must  always 
be  practically  infinitesimal,  and  quite  below  that  needed  for 
detection  by  ordinary  methods.  But  the  rapid  rate  of  change 
of  the  transition-form,  and  the  expulsion  of  a  particles  which 
accompanies  the  change,  afford  the  means  whereby  these 
transition-forms  can  readily  be  detected  and  studied,  although 
only  present  in  so  small  quantity  that  no  other  evidence  of 
their  existence  is  manifested. 

Radio-activity,  as  the  manifestation  of  atomic  disintegra- 
tion, has  thus  introduced  us  to  a  whole  series  of  new  unstable 
elements  which  at  present  find  no  place  in  the  periodic  table, 
and  of  which,  except  in  the  single  case  of  radium,  we  posses  but 
little  chemical  knowledge.  After  the  initial  stage  is  passed 
the  evolution  proceeds  rapidly,  and  passes  from  stage  to  stage 
so  quickly  that  almost  all  we  know  at  present  of  the  inter- 
mediate forms  is  derived  from  the  energy-phenomena  which 
mark  their  appearance  and  destruction.  Yet  these  pheno- 
mena— viz.,  the  expulsion  of  rays — are  so  characteristic  and  so 
fitted  for  exact  study  that  our  knowledge  is  by  no  means 
necessarily  scanty,  although  it  is  of  a  kind  not  usually  asso- 
ciated with  ordinary  matter.  This  appears  at  once  when  the 
means  of  identifying  and  distinguishing  the  different  types  of 
metabolons  from  one  another  are  compared  with  those  em- 
ployed in  the  case  of  the  stable  atoms.  For  the  latter,  atomic 
weight,  spectrum  reaction,  and  general  chemical  and  physical 


THEORY  OF  ATOMIC  DISINTEGRATION.  125 

nature  are  the  distinctive  criteria  employed.  In  the  case  of 
the  metabolons,  physical  and  chemical  nature  can  sometimes 
be  fairly  completely  studied.  But  the  most  valuable  criterion, 
which  is  universally  applicable,  is  the  rate  of  change  as  defined 
by  the  radio-active  constant  A,  the  proportion  of  the  total 
changing  per  second. 

A  relation  perhaps  more  generally  useful  than  the  radio- 
active constant  is  its  reciprocal  I/A,  which  has  a  very  interest- 
ing physical  significance.  It  represents  the  average  life  of  the 
metabolon  in  seconds,  and  affords  a  more  concrete  mental 
picture  than  the  rate  of  change.  Thus,  for  the  thorium 
emanation  the  average  life  of  the  metabolon  is  87  seconds,  for 
the  radium  emanation  5 '3  days  (see  Appendix,  Chapter  IX.) 
The  average  life  of  a  metabolon  may  be  compared  with  the 
atomic  weight  in  the  case  of  a  stable  atom  as  a  constant  well 
suited  for  its  experimental  identification.  It  may  be  pointed 
out  that  the  actual  life  of  the  different  atoms  of  the  same 
unstable  element  has  all  values  between  zero  and  infinity. 
Some  break  up  during  the  first  second  of  existence,  and,  since 
only  a  fraction  of  the  total  changes  per  second,  the  quantity 
is,  theoretically,  never  reduced  to  zero,  and  some  persist 
indefinitely.  This  constitutes  the  first  difference  in  properties 
between  the  individual  atoms  of  the  same  element  that  has 
ever  been  discovered.  It  may  be  likened  to  the  individual 
differences  of  velocity  that  exist  between  the  molecules  of  a 
gas  at  constant  temperature,  according  to  the  kinetic  theory. 
This  is  important,  as  it  suggests  the  question  whether  all 
atomic  properties  are  not  really  average  properties,  the 
individual  atoms  continually  passing  with  great  rapidity 
through  phases  varying  widely  among  themselves  in  chemical 
and  physical  nature. 

Of  all  possible  groupings  of  matter,  the  atoms  of  the 
periodic  law  probably  represent  only  a  selected  number — 
viz.,  the  forms  with  longest  life — which  exist  to-day  because 
they  have  survived  a  long  process  of  evolution  in  which 
those  physically  unfit  have  disappeared.  The  transition- 
forms  already  spoken  of  represent,  on  the  other  hand,  the 
elementary  forms  of  matter  physically  unfit  to  survive,  but 
which  are  brought  within  our  powers  of  knowledge  because 
they  constitute  the  temporary  halting  places  through  which 


126  RADIO-ACTIVITY. 

matter  is  passing  in  a  scheme  of  slow  continuous  evolution 
from  the  heavier  to  the  lighter  forms.  The  original  radio- 
active elements  may  be  described  as  the  connecting  links 
between  the  two  classes,  partaking  of  the  properties  of  each. 
They  are  changing,  but  so  excessively  slowly  that  some  still 
survive.  This  gives  us  an  insight  into  the  reason  of  the  stability 
of  the  elements.  Matter  has  passed  to  its  present  position  of 
apparent  immutability  by  a  long  process  of  natural  selection. 
The  elements  known  to  the  chemist  are  stable  because  they 
exist  and  have  survived.  On  the  other  hand,  it  is  now  possible 
to  examine  some  excessively  unstable  forms  of  matter.  Radio- 
activity might  be  denned  as  the  science  of  the  ephemeral 
-elements. 


CHAPTER  IX. 


THE  RADIO-ACTIVE  PEOPERTIES  OF  RADIUM. 

Intensity  and  Permanence  of  the  Activity  of  Radium.— Chemical  Actions 
of  the  Radium  Rays. — Physiological  Actions. — The  Emanation  of 
Radium. — The  Imparted  Activity  of  Radium.— Rate  of  Decay  of  the 
Emanation. — ^-Radiations  Produced  intheLast  Stages  of  the  Disinte- 
grations.— Summary  of  Radio-active  Changes  of  Radium. — Evidence 
of  tlie  Complexity  of  the  Changes  giving  rise  to  the  Imparted  Activity. — 
"Induced  Activity  of  Radium  with  Slow  Rate  of  Dissipation." — The 
Actinium  Emanation. — Charge  Carried  by  the  Eminatiom. — Methods 
of  Radio-active  Analysis  of  Minerals. 

APPENDIX. — Table  I.,  Course  of  Disintegration  of  Urjnium,  Thorium, 
Radium,  Actinium. — Table  II.,  Radio-active  Constants  and  the 
Values  of  the  Average  Life  of  the  Unstable  Elements. 

The  main  interest  attaching  to  radium  is  on  account  of  the 
intensity  and  permanence  of  its  radio-activity.  We  have  seen 
that,  according  to  the  disintegration  theory,  the  product  of  the 
intensity  of  the  activity  into  the  length  of  time  it  persists  may 
be  expected  to  be  similar  in  all  cases.  The  more  intensely 
active  any  type  of  matter  is  the  more  rapidly  will  it  tend  to 
exhaustion,  and  therefore  the  intensely-active  or  rapidly- 
changing  forms  can  only  exist  at  the  present  time  if  they  are 
being  continuously  reproduced  in  the  slower  change  of  a  more 
permanent  element.  Radium  occupies  a  very  interesting 
position.  Its  rate  of  change  is  fast  enough  to  cause  its  radio- 
activity to  be  very  remarkable.  The  energy  emission  is  so 
great  that  it  is  manifested  in  ways  other  than  the  direct 
radiation.  The  preparations  continuously  generate  heat  and 
produce  marked  chemical  actions.  -On  the  other  hand,  the 
change  is  so  slow  that  the  substance  does  not  measurably 
exhaust  itself  in  a  short  term  of  years,  and  appears  at  first 


128  RADIO- ACTIVITY. 

sight  to  be  not  changing  at  all.  In  brief,  the  position  of 
radium  is  intermediate  between  that  of  a  slow-changing 
parent  element  like  uranium  or  thorium  and  a  rapidly- 
changing  transition-form  like  uranium  X  or  thorium  X.  Its 
rate  of  change  is  just  slow  enough  not  to  be  obvious  in  a  short 
period  and  yet  sufficiently  rapid  for  the  change  to  be  detectable 
by  direct  observation,  as,  for  example,  by  the  production  of 
helium,  if  methods  of  the  utmost  delicacy  are  employed  and 
a  moderate  lapse  of  time  allowed.  Or,  to  consider  another 
aspect  of  the  question,  the  rate  of  change  is  so  rapid  that  only 
an  infinitesimal  quantity  can  accumulate  in  any  mineral  in 
which  it  is  being  produced,  and  yet  is  sufficiently  slow  that 
when,  by  the  expenditure  of  infinite  patience,  as  in  Mme. 
Curie's  research,  tons  of  mineral  are  extracted,  a  sufficient 
weight  can  be  accumulated  to  come  within  the  range  of 
ordinary  chemical  and  spectroscopic  methods  of  recognition. 
For  we  shall  see  as  we  proceed  that  it  is  as  certain,  in  the 
present  state  of  knowledge,  that  radium  is  being  continuously 
reproduced  in  minerals  containing  it  as  that  the  emanation  is 
being  produced  by  radium,  or  helium  by  the  emanation.  The 
element  has  thus  been  the  means  of  opening  out  an  entirely 
fresh  field  of  research,  in  which  the  requirements  of  the  theory 
of  atomic  disintegration  have  been  subjected  to  direct  verifica- 
tion by  ordinary  physical  and  chemical  methods. 

Chemical  Actions  of  the  Radium  Eays. — The  radiations  from 
radium  are  so  much  more  powerful  than  those  of  uranium  and 
thorium  that  they  produce  many  effects  not  strong  enough  to 
be  appreciable  in  the  latter  cases.  They  are  able,  for  example, 
to  bring  about  many  chemical  changes.  The  most  important 
actions  noticed  are  the  conversion  of  oxygen  into  ozone 
(Demargay),  the  conversion  of  yellow  phosphorus  into  the  red 
variety  (Becquerel),  the  decomposition  of  water  into  hydrogen 
and  oxygen  (Giesel),  and  the  colouration  of  glass  violet  or 
brown  (Curie).  In  the  conversion  of  oxygen  to  ozone  and  the 
decomposition  of  water  a  large  absorption  of  energy  occurs, 
and  it  was  from  evidence  of  this  nature  that  the  large  amount 
of  energy  continuously  emitted  Toy  radium  was  first  recognised. 
A  gramme  of  pure  radium  bromide  in  solution  in  water  will 
evolve  about  10  cubic  cm.  of  mixed  hydrogen  and  oxygen  pet- 
day,  which  corresponds  to  the  production  of  20  gramme-calories 


RADIO-ACTIVE  PROPERTIES   OF  RADIUM.  129 

of  heat  per  day.  This  evolution  goes  on  steadily  month  after 
month  at  constant  rate.  An  unexplained  point  about  this 
reaction  is  that  there  is  always  a  slight  excess  of  hydrogen 
over  the  composition  required  to  re-form  water.  This  has 
been  variously  accounted  for,  but  at  first  seemed  to  be  due  to 
the  oxygen  being  used  up  in  effecting  oxidations.  In  recent 
experiments,  however,  in  which  the  possibility  of  this  effect 
has  been  avoided,  the  excess  of  hydrogen  still  appeared. 
Giesel  has  stated  that  solutions  of  radium  bromide  generate 
bromine  as  well  as  hydrogen  and  oxygen,  and  this,  if  correct, 
would  possibly  account  for  the  excess  of  hydrogen.  But  in 
the  experiments  mentioned  no  trace  of  bromine  was  detected, 
and  the  genesis  of  the  excess  hydrogen  is  still  unelucidated. 

There  seems  some  doubt  also  about  the  actual  production  of 
ozone  from  oxygen.  Certainly,  in  many  cases  it  is  not  formed 
even  where  it  might  reasonably  be  expected.  But  there  is  no 
doubt  that  oxygen  under  the  action  of  the  rays  from  radium, 
or  from  the  emanation,  is  extremely  active  and  can  effect 
many  oxidations  at  ordinary  .temperatures  which  normally  it 
is  quite  unable  to  do.  Mercury  is  converted  into  the  yellow 
oxide  and  carbon  dioxide  is  produced  if  carbonaceous  matter  is 
present  when  the  emanation  is  stored  in  an  atmosphere  of 
oxygen  (Ramsay  and  Soddy,  Proc.  Roy.  Soc.,  1903,  72,  p.  204). 
One  may  suppose  that  the  oxygen  molecule  is  dissociated  by 
the  rays,  and  the  single  atoms  of  oxygen  effect  these  oxidations 
in  a  manner  analogous  to  "nascent  oxygen."  If  an  atom  and 
a  molecule  combined,  ozone,  O3,  would  be  produced,  but  this 
certainly  does  not  always  occur.  The  production  of  ozone,  if 
real,  may,  therefore,  be  considered  as  merely  a  special  effect  of 
the  general  case — the  formation  of  atomic  oxygen.  The  effect 
of  the  rays  on  the  photographic  plate  is,  of  course,  to  be  classed 
under  the  chemical  reactions  of  the  rays.  Various  fluorescent 
substances,  as  barium  platinocyanide  and  zinc-blende,  are 
changed  by  prolonged  action  of  the  rays,  and  then  become 
more  or  less  insensitive.  Willemite  is  probably  one  of  the 
most  lasting  fluorescers  we  possess,  and  does  not  deteriorate 
appreciably  even  after  long  exposure. 

Physiological  Ejects  of  the  Radium  Bays. — Powerful  prepara- 
tions of  radium,  if  allowed  to  act  at  close  quarters  on  the  skin 
for  a  sufficiently  prolonged  period,  produce  ulcerated  wounds 

I 


130  RADIO- ACTIVITY. 

which  are  very  difficult  to  heal.  The  effect  has  not  yet  been 
exactly  studied,  but  it  would  seem  that  the  /3-rays  are  mainly 
operative  in  producing  this  effect  as  ordinarily  observed.  On 
the  other  hand,  the  a-rays  would  probably  be  found  to  be 
even  more  harmful  than  the  /3-rays  if  precautions  were 
taken  against  their  being  absorbed  before  reaching  the 
skin.  This  is  a  condition  not  easy  to  observe,  although  it  is 
possible  that  if  the  radium  were  enclosed  in  a  celluloid  capsule 
some  of  the  emanation  would  find  its  way  through  and  produce 
its  a-rays  in  direct  contact  with  the  skin  (see  footnote  below). 

The  most  interesting  physiological  effect  of  the  rays  of 
radium  is  on  the  retina  of  the  eye.  If,  after  the  eye  has  been 
rested  in  absolute  darkness  for  some  minutes,  a  quantity  of  a 
radium  compound  is  brought  suddenly  near  to  the  upper  part 
of  the  forehead,  a  diffuse  light  seems  to  fill  the  back  of  the 
head  even  if  the  eyelids  are  closed  and  the  radium  is  cased  in 
lead  or  steel.  This  effect  was  discovered  by  Giesel,  who 
explains  it  as  being  due  to  the  fluorescence  of  the  centre  of 
the  eye.  Blind  people  with  the  retina  uninjured  experience 
the  effect,  but  if  the  retina  is  diseased  the  effect  is  not 
obtained.  The  test  may  prove  of  some  use  in  obtaining 
information  as  to  the  cause  of  blindness. 

Radium  Emanation. — Radium  resembles  thorium  very  closely 
in  the  character  of  its  radio-active  disintegration  products. 
It  gives  a  gaseous  emanation  possessing  an  almost  completely 
similar  physical  and  chemical  nature  to  the  thorium  emanation, 
resisting  absorption  by  all  known  reagents,*  condensing  and 
becoming  non-volatile  at  low  temperatures,  and  exhibiting  to  an 
even  more  marked  degree  than  the  thorium  emanation  the 
property  of  being  retained  by  the  dry  solid  compounds  pro- 
ducing it,  and  of  being  liberated  therefrom  by  heat  and 
solution.  It  is  distinguished  by  a  much  slower  rate  of  change. 
Dorn  (AWc.  der  Naturforsch.  Ges.  fur  Halle,  1900)  first  recog- 
nised the  existence  of  the  radium  emanation  by  testing  a 
sample  of  radio-active  barium  prepared  from  pitchblende  in  a 
manner  similar  to  that  already  described  by  Eutherford  for 

*  M.  and  Mme.  Curie  have  described  some  experiments  which,  when 
interpreted,  seem  to  indicate  that  celluloid  has  the  power  of  absorbing 
the  emanation  and  retaining  it.  It  would  be  interesting  if  further 
experiments  could  be  carried  out  on  this  point. 


RADIO-ACTIVE  PROPERTIES   OF  RADIUM.  131 

thorium,  and  pointed  out  that  its  activity  lasted  a  much  longer 
time  than  in  the  case  of  the  thorium  emanation.  It  can  still 
be  detected  several  weeks  after  it  has  been  separated  from 
the  radium  producing  it. 

Owing  to  the  comparatively  slow  rate  of  disappearance,  the 
radium  emanation,  when  it  does  not  succeed  in  escaping, 
accumulates  in  the  compound  producing  it  to  a  much  more 
marked  extent  than  in  the  case  of  thorium.  It  has  been  shown 
that  the  amount  of  any  transition-form  capable  of  accumulating 
reaches  an  equilibrium  value  when  the  amount  produced  per 
second  equals  the  amount  disintegrating  per  second.  By 
definition  A  is  the  fraction  of  the  total  amount  disintegrating 
per  second.  If  NO,  represents  the  equilibrium  quantity  of 
emanation,  and  q0  the  quantity  produced  per  second,  q0  =  ANo, 
or  N0/2o=l/A.  A  for  the  thorium  emanation  is  1/87,  and  for 
the  radium  emanation  1/463,000,  or  6,000  times  smaller. 
Hence,  if  compounds  of  radium  and  thorium  are  taken  and 
kept  under  conditions  in  which  the  radio-active  emanation 
does  not  escape  (most  conveniently  in  the  form  of  solutions 
in  stoppered  vessels)  for  a  sufficient  length  of  time  for  the 
equilibrium  point  to  be  reached  in  each  case,  the  maximum 
amount  of  radium  emanation  accumulating  in  the  bottle  will 
be  463,000  times  the  amount  produced  per  second,  while  in 
the  case  of  the  thorium  emanation  the  maximum  amount  will 
be  only  87  times  the  amount  produced  per  second.  These 
relations  have  been,  quantitatively  verified  by  actual  experi- 
ments (Rutherford  and  Soddy,  Phil.  Mag.,  1903,  VI.,  5.,  p.  450). 

Xearly  all  dry  solid  radium  compounds  give  out  very  little 
emanation  until  they  are  warmed  or  dissolved,  when  there 
occurs  a  sudden  evolution  of  the  stored-up  emanation  several 
thousand  times  greater  than  that  ordinarily  given  by  the  cold 
dry  salt.  If  this  is  swept  away  by  a  current  of  gas  or  extracted 
by  a  mercury  pump  and  stored,  it  will  be  found  that  the  gas 
is  spontaneously  luminous  in  the  dark  and  remains  so  for 
several  weeks,  the  luminosity  continuously  diminishing  till 
finally  it  completely  disappears.  Speaking  more  strictly,  the 
luminosity  is  not  due  to  the  gas  itself  but  to  the  fluorescence 
of  the  walls  of  the  containing  vessel.  Once  the  emanation  is 
extracted  in  this  manner  from  any  given  sample  of  radium, 
no  appreciable  further  quantity  can  be  then  obtained.  As  the 


132  RADIO-ACTIVITY. 

activity  of  the  gas  extracted  diminishes  with  lapse  of  time,  a 
fresh  crop  of  emanation  is  manufactured  by  the  radium,  the 
total  radio-activity  due  to  emanation  being  at  all  times  con- 
stant. The  explanation  is  the  same  as  in  the  case  of  thorium 
and  thorium  X,  and  the  rate  of  decay  of  the  activity  of  the 
radium  emanation  is  similar  to  that  of  thorium  X,  being 
reduced  to  half  the  initial  value  after  the  lapse  of  about  3'7 
days.  The  radium  emanation,  although  otherwise  similar 
in  general  properties  to  the  thorium  emanation,  escapes  into 
the  air  with  even  less  facility  from  solid  compounds.  In  both 
cases,  so  far  as  we  know,  the  emanation  escapes  instan- 
taneously from  the  compounds  in  aqueous  solution.  The 
emanation  is  being  continuously  produced  by  radium  at  a 
constant  rate,  in  exactly  the  same  manner  as  thorium  X  is 
produced  from  thorium.  It  depends  entirely  on  the  physical 
conditions  whether  the  emanation  escapes  or  is  accumulated. 
The  amount  produced  is,  however,  always  constant  and  inde- 
pendent of  the  conditions. 

The  Imparted  Activity  of  Radium. — M.  and  Mme.  Curie 
(Comptes  Rendus,  1899,  129,  p.  823)  discovered  that  radium 
has  the  power  of  conferring  radio-activity  on  its  surroundings, 
and  this  phenomenon  was  later  studied  by  Curie  and  Debeirne 
(Cvmptes  Eendus,  1901,  132,  p.  768;  1902,  133,  pp.  276  and 
931).  For  a  long  time  these  investigators  held  the  view  that 
the  radio-activity  was  "induced"  in  the  inactive  matter  by 
the  radiations  of  the  radium.  But  it  has  now  been  admitted 
that  the  "  induced  "  or  imparted  activity  of  radium  is  caused 
by  the  emanation  in  a  precisely  analogous  manner  to  that  of 
the  allied  phenomena  in  the  case  of  thorium.  The  imparted 
radio-activity  from  radium  decays  more  rapidly  than  that  from 
thorium,  falling  to  half  value  in  28  minutes,  but  possesses  the 
same  property  as  the  latter  of  travelling  in  the  electric  field 
and  being  concentrated  entirely  on  the  negative  electrode. 
The  matter  causing  the  radium- imparted  activity  possesses 
interesting  features,  in  that  its  radio-activity  is  derived  from 
three  separate  and  successive  disintegrations.  In  a  first  con- 
sideration only  the  last  of  the  three  changes  need  be  considered, 
as  this  is  the  one  usually  most  important.  The  rate  of  decay 
given  above  for  the  imparted  activity  of  radium — viz.,  to  half- 
value  in  28  minutes  (A  =  4-1  xlO~4)— applies  to  the  last 


RADIO-ACTIVE  PROPERTIES  OF  RADIUM. 


133 


disintegration.  The  stored-up  emanation  find  the  imparted 
activity  it  produces  in  the  mass  of  the  dry  solid  compounds  of 
radium  cause  the  greater  part  of  the  radio-activity.  When  they 
are  removed,  by  keeping  the  radium  in  solution  in  the  open  air 
for  some  hours,  the  activity  of  the  radium  compound  after 
evaporation  to  dryness  is  very  much  enfeebled,  but  slowly 
recovers  as  the  emanation  and  imparted  activity  is  regene- 
rated in  the  compound.  A  non-separable  radio-activity  of  the 
radium  is  retained  after  solution,  or  any  other  treatment, 
and  constitutes  about  25  per  cent,  of  the  total  activity.  As 
in  the  case  of  thorium  and  uranium,  it  comprises  only 
a- radiations. 

This  enfeeblement  of  the  activity  of   radium  preparations 
through  damp  and  the  corresponding  increase  of  activity  of  dry, 


1      2     3     4      5     6     7     8     9    10    11    12    13    14    15   16 
Time  in  Days. 

FIG.  31. 


solid  salts  from  the  moment  of  their  preparation  from  their  solu- 
tion was  known  many  years  before  the  changes  were  connected 
with  the  escape  and  re-accumulation  respectively  of  the 
radio-active  emanation  (Giesel,  Wied.  Ann.,  1899,  VIA.,  p.  91 ; 
M.  and  Mme.  Curie,  Comptes  Eendus,  1902,  134,  p.  85). 

Fig.  31  shows  the  curves  obtained  (Rutherford  and  Soddy, 
Phil.  Mag.,  1903,  VI.,  5,  p.  455)  for  the  recovery  of  the 
activity  of  radium  compounds  and  the  decay  of  the  activity 
of  the  emanation.  They  are  in  every  respect  analogous  to 
those  given  for  the  case  of  thorium  and  thorium  X  (see  Fig.  26). 
In  curve  A  the  activity  regained,  between  the  minimum  and 


134  RADIO-ACTIVITY. 

maximum  activities,  is  taken  as  100.  Curve  B  was  obtained 
by  storing  the  emanation  mixed  with  air  in  a  gasholder  over 
mercury,  and  removing  from  day  to  day  an  aliquot  portion, 
which  was  introduced  into  a  testing  vessel  provided  with  an 
internal  electrode,  so  that  the  ionisation  current  through 
the  gas  due  to  the  emanation  could  be  determined.  The 
current  is  measured  immediately  after  the  introduction  of 
the  emanation,  for  the  activity  rises  rapidly  at  first  owing 
to  the  production  of  the  matter  causing  the  imparted  activity 
from  the  emanation.  It  will  be  seen  that  the  activity  of  the 
emanation  falls  to  half  value  in  about  3 '7  days,  and  the  value 
of  A  is  about  2-2  x  10~6.  M.  Curie  (Comptes  liendus,  1902, 
135,  p.  857)  found  that  the  penetrating  rays  from  a  sealed  glass 
tube  containing  the  radium  emanation  decayed  to  half  value 
in  four  days,  and  this  constitutes  a  separate  determination  of 
the  decay  of  the  emanation.  For  although  the  rays  in  question 
do  not  come  from  the  emanation,  which  we  know  gives  no 
/?-rays,  but  from  the  imparted  activity,  it  can  be  shown  by  the 
disintegration  theory  that  the  rate  of  decay  after  the  first  few 
hours  must  be  that  of  the  radio-activity  of  the  emanation  in 
the  tube.  For  we  have  seen  in  the  case  of  thorium  (p.  Ill) 
that  the  radiating  power  of  the  matter  causing  the  imparted 
activity  deposited  from  the  emanation  is  proportional  to  the 
radio-activity  of  the  emanation  producing  it,  and  as  the  latter 
decays  the  imparted  activity  diminishes  correspondingly.  By 
subjecting  the  sealed  tubes  during  the  experiments  to  tempera- 
tures varying  between  450°C.  and  -  180°C.  it  was  shown  that 
the  rate  of  decay,  and  therefore  the  radio-active  constant  A,  is 
completely  unaffected  by  change  of  temperature. 

The  disturbing  effects  of  the  later  products  of  the  disin- 
tegration, as,  e.g.,  the  production  of  the  imparted  activity, 
upon  the  course  of  the  earlier  ones,  which  has  already  been 
referred  to  for  thorium,  is  much  more  marked  in  the  case 
of  radium.  Curve  A  (Fig.  29)  was  obtained  by  working 
with  radium  that  had  been  kept  in  solution  some  hours 
before  evaporating  to  dryness.  If,  however,  the  dry  salt 
containing  its  maximum  charge  of  occluded  or  stored-up 
emanation  is  dissolved  in  water  and  immediately  evaporated 
to  dryness,  the  emanation  completely  escapes,  but  the 
imparted  activity  produced  by  it  in  the  compound  remains 


RADIO-ACTIVE  PROPERTIES  OF  RADIUM.  135 

with  the  radium.  The  activity,  instead  of  being  reduced 
by  this  treatment  to  the  25  per  cent,  constant  minimum, 
is  only  reduced  to  about  60  per  cent,  of  the  maximum 
value.  But  a  very  rapid  decay  to  nearly  the  25  per  cent, 
value  occurs  in  the  course  of  the  first  three  or  four  hours 
after  the  evaporation.  This  is  due  to  the  imparted  activity 
produced  in  the  compound  by  the  stored-up  emanation  rapidly 
decaying  when  the  latter  is  removed.  After  a  few  hours 
the  activity  again  commences  to  increase,  and  the  subsequent 
progress  of  the  recovery  is  the  same  as  in  curve  A.  The 
converse  of  this  phenomenon  is  to  be  found  in  the  rapid 
increase  of  activity  for  the  first  few  hours  when  the  emanation 
is  introduced  into  a  fresh  vessel.  If  the  central  electrode 
is  made  negative  with  respect  to  the  vessel,  the  imparted 
activity  is  concentrated  solely  upon  it,  as  in  the  case  of  that 
from  thorium.  After  the  introduction  of  the  emanation  the 
activity  steadily  rises,  very  rapidly  indeed  for  the  first  few 
minutes,  and  then  slowly  for  three  or  four  hours  until  it  attains 
a  maximum  nearly  twice  as  great  as  at  first.  If  at  any  time 
the  central  electrode  is  removed,  the  imparted  activity  upon  it 
will  be  found  to  account  for  the  increase  of  the  value  of  the 
activity  above  the  original  value.  Or,  the  emanation  may  be 
suddenly  blown  out  at  any  stage,  when  the  current  through 
the  gas  is  reduced  to  the  proportion  of  the  total  activity  due 
to  the  imparted  matter  on  the  negative  electrode. 

f}-mys. — Kadium  furnishes  valuable  evidence  of  the  stage  of 
the  disintegration  at  which  the  y8-ray  is  produced.  It  will  be 
recalled  that  in  the  case  of  uranium  the  a-radiation  is  pro- 
duced in  the  first  change  of  uranium  into  uranium  X  and  the 
/3-radiation  in  the  subsequent  disintegration  of  uranium  X. 
The  cases  of  thorium  and  radium  are  less  simple.  With  regard 
to  the  former  a  complete  separation  of  the  matter  causing  the 
imparted  activity  from  thorium  X  has  not  yet  been  effected. 
Now,  the  imparted  activity  gives  /3  as  well  as  a- rays,  and  it  is 
possible  that  the  /3-radiation  of  thorium  X  is  due  entirely  to 
this  source.  The  emanation  (compare  p.  101)  is  known  to 
give  only  a-rays,  and  the  same  is  true  of  the  non -separable 
activity  of  thorium.  The  lower  the  emanating  power  of  a 
thorium  compound — that  is,  the  more  the  emanation  is  retained 
by  the  compound  to  produce  the  imparted  activity  within  it — 


136  RADIO-ACTIVITY. 

the  greater  the  proporton  of  /?- radiation  from  the  compound. 
Although  it  has  not  been  definitely  proved,  the  facts  are  in 
agreement  with  the  possibility  that  the  /^-radiation  of  thorium 
occurs  only  in  the  last  stage  of  the  disintegration,  as  in  the 
case  of  uranium. 

In  the  case  of  radium  the  evidence  is  more  definite.  If  the 
radium  emanation  is  introduced  into  a  vessel  the  walls  of  which 
are  thick  enough  to  completely  absorb  the  a-rays  and  thin  enough 
to  allow  the  /?-rays  to  pass  through,  it  is  found  that  at  first  there 
is  no  external  radiation  from  the  vessel — i.e.,  there  are  no 
/5-rays  produced  (Curie,  Comptes  Eendus,  1902,  135,  p.  857  ; 
Eutherford  and  Soddy,  Phil.  Mag.,  1903,  VI.,  5,  p.  456).  In  a 
short  time  after  the  introduction  of  the  emanation,  an  external 
or  /^-radiation  is  produced  from  the  vessel,  and  this  rapidly  and 
regularly  increases,  attaining  a  practical  maximum  in  three  or 
four  hours.  If  the  emanation  is  then  swept  out  of  the  vessel 
by  a  blast  of  air,  the  value  of  the  external  radiation  from 
the  vessel  is  not  suddenly  affected,  but  commences  to  decay 
regularly,  falling  to  half  value  in  30  minutes,  and  decreasing 
practically  to  zero  in  three  or  four  hours.  Thus  the  radium 
emanation  gives  only  a-rays.  The  /?-rays  are  produced  after 
it  has  changed  into  the  matter  causing  the  imparted  activity. 
Since  the  decay  of  the  ^-radiation  of  the  latter  is  regular, 
falling  to  half  value  in  30  minutes,  it  follows  that  the  /^-rays 
are  produced  in  the  later  changes  of  this  matter,  for  it  will  be 
shown  later  that  the  first  change  suffered  by  the  matter  causing 
the  imparted  activity  is  very  rapid,  and  the  time-constant 
characteristic  of  the  change  of  the  /2-radiation — to  half  value 
in  30  minutes— is  that  of  the  last  change  that  can  be  experi- 
mentally traced.  Hence,  in  both  uranium  and  radium,  and 
probably  also  in  thorium,  the  /3-rays  only  appear  at  the  last 
stage  of  the  disintegration.  Now  the  y-rays  accompany  the 
/3-rays,  and  are  always  produced  with  them  (Rutherford). 
Hence  we  must  regard  the  a-ray  as  being  the  most  charac- 
teristic feature  of  radio-activity,  and  the  /?  and  y-rays  as  less 
important  and,  in  point  of  time,  secondary.  Polonium,  for 
example,  gives  only  a-rays,  showing  that  the  @  and  y-rays  are 
not  essential  to  the  phenomenon.  Atomic  disintegration,  in 
the  majority  of  instances  we  know  of,  when  the  separate 
stages  are  considered  each  by  itself,  proceeds  with  the  expulsion 


RADIO-ACTIVE  PROPERTIES   OF  RADIUM.  137 

of  a-rays  alone.  The  important  issue  raised  as  to  how  the 
equivalence  of  electric  charges  is  maintained,  if  positively 
charged  particles  are  being  continually  expelled  alone,  will  be 
considered  in  the  last  chapter. 

The  foregoing  results  may  be  briefly  summarised.  The 
radio-activity  of  a  compound  of  radium  that  has  been  kept  for 
some  time  in  the  dry  solid  state — a  sufficient  time  to  attain  its 
maximum  activity — is  made  up  us  follows : — (1 )  A  non-separable 
activity,  comprising  25  per  cent,  of  the  total  a-radiation ;  (2)  a 
part  due  to  the  occluded  emanation,  comprising  about  40  per 
cent,  of  the  total  a-radiation ;  (3)  the  imparted  activity,  com- 
prising about  35  per  cent,  of  the  total  a-radiation  and  all  the 
(3  and  y-radiation.  If  the  compound  is  dissolved  in  water 
and  immediately  evaporated  to  dryness,  or  if  heated  and 
cooled,  the  activity  of  the  radium  is  reduced  and  is  now  due 
to  (1)  and  (3),  comprising  about  60  per  cent,  of  the  a-rays  and 
all  the  fi  and  y-rays.  On  keeping  for  five  hours  its  activity 
decays  further  and  then  comprises  only  (1),  viz.,  25  per  cent, 
of  the  a  rays  and  no  /3  or  y-rays.  The  emanation  given  off 
on  heat  or  solution  at  first  comprises  40  per  cent,  of  the  total 
a-radiation  and  gives  no  ft  or  y-rays.  If  kept  for  five  hours 
in  a  closed  vessel  it  then  comprises  (3)  as  well  as  (2),  and 
represents  75  per  cent,  of  the  a  and  all  the  /?  and  y-radiation. 
If  now  the  emanation  is  blown  out  of  the  old  vessel  into  a  new 
vessel,  the  latter  will  again  show  the  40  per  cent,  a-radiation 
due  to  the  emanation  alone,  whereas  the  old  vessel  will  retain 
35  per  cent,  of  the  a-radiation  and  all  the  fi  and  y-radiation.  A 
complete  separation  of  the  constituents  (1),  (2),  (3)  has  thus 
been  simply  effected.  In  five  hours  the  activity  of  the  old 
vessel  almost  completely  disappears,  whereas  the  new  vessel 
in  which  the  emanation  has  been  stored  again  shows  an 
increased  a  and  all  the  J3  and  y-radiation.  Now,  consider  the 
radium  preparation  and  the  stored  emanation  over  a  further 
period  of  one  month.  During  this  interval  the  radium 
gradually  regains  its  radiating  power,  and  the  vessel  in  which 
the  emanation  is  stored  correspondingly  loses  its  radiating 
power.  In  these  changes  the  a,  ft  and  y  rays  now  grow  and 
decay  together.  At  the  end  of  a  month  the  radium  is  again 
in  its  initial  condition  of  maximum  activity,  and  the  activity 
of  the  separated  products  is  practically  nil.  The  whole  of 


138  RADIO-ACTIVITY. 

this  complicated  series  of  changes,  ever}7  step  of  which  is 
predicted  by  the  disintegration  theory,  is  followed  in  the 
minutest  detail  when  the  simple  experiment  of  heating  a  radium 
compound  is  investigated  in  the  laboratory.  The  above 
example  furnishes  a  good  instance  of  the  principle  of  the  con- 
servation of  radio-activity. 

Evidence  of  the  Complexity  of  the  Changes  Causing  the  Imparted 
Activity  of  Radium  and  Thorium. — The  curve  of  decay  of  the 
radium-imparted  activity  is  very  irregular  (Kutherford  and 
Miss  Brooks,  Phil.  Mag.,  1902,  VL,  4,  p.  18).  It  consists 
of  (1)  a  very  rapid  decrease  to  possibly  20  per  cent,  of  the 
original  value  in  the  course  of  a  few  minutes  ;  (2)  a  period  of 
a  few  minutes  of  slow  change,  which  is,  under  some  circum- 
stances, a  slight  increase,  and,  under  others,  a  slight  decrease ; 
(3)  a  final  geometrical  decrease  to  zero,  the  activity  falling  to 
half  value  in  30  minutes.  Now,  under  certain  circumstances,  an 
irregular  curve  can  be  obtained  also  for  the  decay  of  the  thorium  - 
imparted  activity  (Rutherford,  Phys.  Zeit.,  1902.)  If  a  wire 
is  exposed  for  a  short  time  only  to  the  emanation  of  thorium, 
it  is  found  on  removal  that  the  imparted  activity  increases  at 
first  instead  of  decaying,  and  may  rise  in  the  course  of  the 
first  few  hours  to  three  or  four  times  its  initial  value,  from  that 
point  decaying  at  the  normal  rate  to  half  value  in  11  hours. 
So  also  in  the  case  of  radium,  the  initial  rapid  decrease  of  the 
imparted  activity  is  the  more  marked  the  shorter  the  time 
of  exposure  to  the  action  of  the  emanation.  These  effects 
would  be  produced  in  each  case  if  the  disintegration  causing 
the  activity  of  the  imparted  matter,  instead  of  being  single, 
consisted  of  more  than  one  successive  change  whose  combined 
effects  give  rise  to  the  radio-activity. 

It  is  necessary  to  assume  two  changes  in  the  case  of  the 
thorium -imparted  activity,  the  radio-active  constant  of  the 
first  type  being  in  the  neighbourhood  of  2  x  10~4,  so  that  the 
change  is  half  complete  in  about  an  hour.  For  the  last  type 
the  value  of  A  is  1'7  x  10~5,  as  already  mentioned. 

For  the  case  of  radium  it  is  necessary  to  suppose  that  three 
types  are  successively  produced.  The  rate  of  change  of 
the  first  is  very  rapid,  being  comparable  with  that  of  the  thoriurr 
emanation,  and  this  accounts  for  the  initial  rapid  increase 
of  activity  when  the  emanation  is  transferred  to  a  new  vessel, 


RADIO-ACTIVE  PROPERTIES  OF  RADIUM.  139 

as  well  as  for  the  initial  rapid  decay  of  the  imparted  activity 
after  the  emanation  is  removed. 

Most  recently  the  decay  of  the  imparted  activity  of  radium 
has  been  subjected  to  more  detailed  examination  by  Ruther- 
ford (Phil  Mag.,  1904,  VI.,  7,  p.  216)  and  Curie  and  Danne 
(Comptes  Itendits,  1904,  138,  p.  633),  and  both  of  these  inde- 
pendent investigations  have  led  to  identical  conclusions.  The 
course  of  the  decay  is  explained  on  the  disintegration  theory 
if  three  successive  disintegrations  occur  in  the  imparted 
activity  matter  after  its  production  from  the  emanation.  The 
respective  radio-active  constants  are  approximately  as  follows  : 
For  the  first  type,  A  =  4  x  10~3 ;  for  the  second,  A  =  5'4  x  10~4 ; 
and  for  the  third,  A,  =  4*1  x  10~4.  The  first  change  is  half 
completed  in  three  minutes,  and  is  accompanied  by  a  radia- 
tions alone;  the  second  is  half  completed  in  21  minutes,  and  is 
not  accompanied  by  any  radiation ;  the  third  and  last  change  is 
half  completed  in  28  minutes,  and  gives  rise  to  a,  /?  and  y-rays 
(Rutherford).  It  would  seem  also,  from  a  cursory  examination, 
that  the  first  type  in  the  case  of  thorium  changes  into  the 
second  without  the  expulsion  of  rays,  although  this  case  has 
not  yet  been  accurately  studied  from  this  point  of  view. 

Thus,  even  in  these  most  complex  cases,  the  simple  law 
holds  that  a  constant  fraction  of  each  type  changes  in  the 
unit  of  time,  and  the  complexity  is  due  solely  to  the  number 
of  successive  types  produced.  In  addition,  we  are  introduced 
for  the  first  time  to  a  sub-atomic  change,  unaccompanied  by 
radio-activity,  which  is  detected  because  it  is  intermediate 
between  two  other  changes  which  are.  This  should  serve  to 
emphasise  how  dependent  we  are  for  our  knowledge  of  natural 
phenomena  on  the  means  of  experimental  investigation  at  our 
disposal.  Sub-atomic  change  has  been  established  in  the  case 
of  the  radio-elements  on  account  of  the  accompanying  expul- 
sion of  radiant  particles.  The  example  just  considered  shows 
that  this  is  not  necessarily  an  essential  accompaniment  of  the 
process  in  all  cases.  Hence  the  absence  of  radio-activity  in 
the  cases  of  the  inactive  elements  does  not  imply  that  these 
necessarily  are  not  undergoing  change,  but  rather  that  no 
means  are  at  present  available  of  detecting  such  changes 
unless  they  happen  to  be  accompanied  by  the  expulsion 
of  radiant  particles.  There  seems  little  doubt  that  our 


140  RADIO-ACTIVITY 

knowledge  of  the  scheme  of  material  evolution  is  at  present 
only  in  its  infancy,  and  that  when  fully  elucidated  it  will 
be  found  to  embrace  all  the  elements  within  its  scope.  To 
arrive  at  this  result  it  is  not  necessary  to  suppose  that  all  the 
elements  are  really  radio-active,  and  that  those  we  call  inactive 
are  in  reality  so  feebly  active  as  to  be  beyond  the  range  of  the 
present  methods.  It  seems  more  reasonable  to  regard  radio- 
activity as  a  somewhat  special  and  unusual  accompaniment  of 
sub-atomic  change.  Such  changes  without  such  accompaniment 
may  be  occurring  even  in  the  commonest  materials  at  rates 
relatively  rapid  when  expressed  in  cosmical  time-units,  and 
still  have  remained  wholly  unsuspected. 

The '  'Induced  Activity  of  Radium  with  Slow  Rate  of  Dissipation. " — 
The  statements  made  in  the  foregoing  account  of  the  imparted 
activity  of  radium,  that  this  decays  to  zero  in  the  course  of  five 
or  six  hours,  are  not  strictly  accurate.  Mme.  Curie  describes 
in  her  Thesis  a  phenomenon,  under  the  above  title,  which,  under 
ordinary  conditions,  is  only  observed  with  very  active  specimens 
of  radium.  If  the  imparted  activity  is  left  to  itself  it  decays 
regularly  after  the  first  half-hour  in  a  geometrical  progression 
with  the  time  (A  =  4'lxlO~4)  until  it  attains  a  value  only 
Tooiro  th  of  the  initial,  and  then  remains  sensibly  constant  over 
many  months.  Hence,  beyond  the  last  type  of  matter  causing 
the  imparted  activity,  there  is  a  further  very  slowly-changing 
transition -form  produced.  It  has  been  suggested  (Soddy,  Wilde 
Lecture,  1904,  Manchester  Literary  and  Philosophical  Society) 
that  this  type  is  Mme.  Curie's  polonium,  and  the  evidence  in 
favour  of  this  view  will  be  considered  in  the  last  chapter. 

The  Actinium  Emanation. — The  rate  of  decay  of  this  emana- 
tion and  of  the  imparted  activity  it  produces  has  recently  been 
determined  by  Debierne  (Comptes  Rendus,  1904,  138,  p.  411). 
The  activity  of  the  emanation  decays  in  a  geometrical 
progression  with  the  time,  falling  to  half -value  in  3-9 
seconds,  and  the  imparted  activity  produced  from  it  also 
decays  regularly,  falling  to  half-value  in  40  minutes.  The 
value  of  A  in  the  first  case  is,  therefore,  0*17  and  in  the 
second  3x10-*.  The  results  obtained  indicate,  however, 
that  an  intermediate  product  exists  between  these  two 
types,  which  appears  to  be  gaseous  and,  therefore,  of  the 
nature  of  an  emanation,  which  changes  even  more  rapidly  than 


RADIO-ACTIVE  PROPERTIES   OF  RADIUM.  141 

the  first  emanation,  but  apparently  does  not  expel  rays.  The 
evidence  is  as  follows  : — When  the  experiment  described  on 
p.  Ill  (Fig.  29)  is  repeated  for  the  actinium  emanation,  the 
amount  of  activity  imparted  to  the  first  electrode  is  less  than 
to  the  succeeding,  and  the  results  indicate  that  when  first 
produced  the  emanation  imparts  no  activity  to  its  surroundings, 
but  this  power  rapidly  increases  to  a  maximum  and  then  decays 
at  the  same  rate  as  the  activity  of  the  emanation.  The  decay 
of  the  activity  of  the  emanation  itself  shows  no  initial  irregu- 
larity. More  detailed  investigations  must  be  awaited  before 
any  definite  conclusions  can  be  drawn. 

Do  the  Emanations  carry  an  Electric,  Charge?  —  Rutherford 
showed  that  the  thorium  emanation  is  not  affected  by  an 
electric  field,  and  concluded  that  it  was  not  charged  (p.  101). 
He  has  also  shown  that  the  radium  emanation  survives 
prolonged  action  of  the  electric  field  without  being  removed 
from  the  atmosphere  in  which  it  is  present.  Recently 
McClelland  (Phil.  Mag.,  May,  1904,  p.  355)  has  shown  that  no 
charge  is  communicated  to  a  vessel  by  admitting  'he  radium 
emanation  through  a  plug  of  glass  wool,  and  concludes  that  it 
is  unchanged.  All  these  results,  however,  merely  show  that 
the  emanation  particle,  if  charged  when  produced,  loses  its 
charge  like  an  ion  and,  as  is  to  be  expected,  presents  the  same 
essential  properties  of  radio-activity  and  volatility  whether 
charged  or  uncharged.  Since,  on  the  average,  the  radium 
emanation  atom  exists  5-3  days  before  disintegrating,  it  would 
be  remarkable  if  it  persisted  charged  so  long  under  ordinary 
conditions  of  storage.  There  are  theoretical  reasons  for 
expecting  the  atom  to  be  charged  at  the  moment  of  its  pro- 
duction, since  it  is  derived  from  a  neutral  radium  atom  by  the 
expulsion  of  a  positive  ion. 

The  case  of  the  actinium  emanation,  for  which  the  average 
period  of  existence  is  only  5-7  seconds,  therefore  presents 
features  of  great  interest.  The  expected  difference  exists. 
When  produced,  the  atom  of  the  actinium  emanation  appears 
to  carry  a  positive  charge,  which  endows  it  with  certain 
peculiarities  which  are  not  exhibited  by  the  radium  and 
thorium  emanations.  Debierne  (Comptes  Rendu*,  1903,  136, 
pp  446  and  767)  showed  that  the  actinium  emanation  moves 
to  the  negative  electrode  in  an  electric  field,  and  Giesel 


142  RADIO-ACTIVITY. 

(Ber  der  D.  CJiem.  Gesel,  1903,  36,  p.  343)  showed  that  the 
emanation  from  his  "  emanation  substance,"  which  we  have 
seen  is  probably  identical  with  Debierne's  actinium,  shows  the 
same  property.  The  experiments  of  Geisel  may  be  described. 
He  placed  the  active  substance  in  a  metal  cylinder,  open  at 
one  end,  5cm.  to  10cm.  from  a  zinc-blende  screen  negatively 
charged  by  means  of  an  influence  machine.  Under  these 
circumstances  the  particles  of  emanation  are  attracted  to  the 
screen  and  travel  to  it  in  straight  lines,  so  that  a  phosphorescent 
image  of  the  opening  of  the  cylinder  is  formed  on  the  screen. 
From  this  image  "secondary  emanation"  streams  away, lighting 
up  the  screen  surrounding.  A  blast  of  air  now  does  not  affect 
the  phosphorescent  image,  but  only  the  "  secondary  emanation." 
Thus  the  emanation  in  an  electric  field  travels  to  the  negative 
electrode  in  straight  lines  like  a  "ray,"  and  Geisel  proposes 
the  term  "E-ray  "  for  this  phenomenon.  Having  given  up  its 
charge,  it  is  no  longer  controlled  by  the  electric  field,  but  can 
be  blown  away,  so  that  it  seems  probable  that  what  Geisel 
calls  "  secondary  emanation "  consists  of  the  emanation  that 
has  given  up  its  charge,  but  has  not  yet  disintegrated.  The 
theoretical  bearing  of  this  result  will  be  considered  in 
Chapter  XII. 

Methods  of  Experimentally  Recognising  Radium,  Thorium  and 
Actinium,  and  the  Analysis  of  Radio-active  Minerals. — The  three 
distinctive  emanations  and  the  three  equally-distinctive  types 
of  imparted  activity  they  produce  furnish  sufficiently  charac- 
teristic analytical  tests  for  the  presence  or  absence  of  the 
elements  radium,  thorium  and  actinium  in  the  radio-active 
minerals.  To  test  for  radium,  these  should  be  brought  into 
the  state  of  solution  and  kept  in  closed  bottles  for  a  few  days. 
The  gas  above  the  solution  should  then  be  drawn  off  and 
stored  for  10  minutes.  In  this  time  the  thorium  and  actinium 
emanations  will  have  completely  disappeared,  and  if  a  sample 
of  the  gas  blown  into  an  electroscope  discharges  it  the  presence 
of  the  radium  emanation  may  be  inferred.  The  conclusion 
may  be  confirmed  in  two  ways.  In  the  first  place,  if  the 
amount  is  adjusted  to  give  a  moderate  rate  of  leak  in  the 
electroscope,  it  will  be  found  that  the  leak  will  increase 
notably — usually  30  or  40  per  cent. — in  the  first  few  minutes 
after  the  gas  is  introduced.  If  then  the  emanation  is  blown 


RADIO-ACTIVE  PROPERTIES   OF  RADIUM.  143 

out  and  the  rate  of  leak  again  tested  it  will  be  found  to  rapidly 
decrease  to  a  very  small  value  in  the  first  few  minutes.  A 
larger  quantity  of  the  emanation  may  then  he  introduced  and 
allowed  to  remain  some  30  minutes  before  being  blown  out. 
The  activity  remaining  will  again  rapidly  diminish,  but  will 
now  be  quite  noticeable  after  half  an  hour.  If  after  this  period 
it  is  tested  from  time  to  time,  it  will  be  found  to  decay  to  half- 
value  after  the  lapse  of  30  minutes,  to  one-quarter  in  an  hour, 
and  so  on.  The  second  test  for  the  radium  emanation  is  to 
store  the  gas  in  an  air-tight  gas-holder,  and  to  remove  equal 
portions  at  intervals  of  four  days.  Its  power  of  discharging 
the  electroscope  will  persist  noticeably  for  many  weeks, 
diminishing  to  one-half  during  each  interval  of  four  days,  or 
correspondingly  for  longer  or  shorter  intervals.  If  it  fulfils 
these  tests,  the  presence  of  radium  in  the  mineral  may  be  con- 
sidered proved  and  its  quantity  deduced  by  comparative  tests, 
using  a  known  sample  of  Joachimsthal  pitchblende  as  the 
standard. 

To  test  for  thorium,  the  gas  swept  out  of  the  bottle  con- 
taining the  mineral  is  rejected,  and  the  emanation,. allowed  to 
re-accumulate  for  five  minutes,  is  removed  to  a  gas-holder  and 
stored  for  one  minute.  In  this  time  the  actinium  emanation 
completely  disappears,  and  if,  when  a  portion  is  blown  into  an 
electroscope,  it  discharges  it  the  presence  of  the  thorium  emana- 
tion may  be  inferred.  This  may  be  proved  as  follows  :  After 
introduction  of  the  gas  the  discharging  power  will  rapidly  decay 
to  half-value  in  one  minute,  to  quarter-value  in  two  minutes, 
and  so  on,  and  in  five  minutes  will  have  completely  disappeared. 
No  appreciable  amount  of  thorium-imparted  activity  is  pro- 
duced under  these  circumstances.  To  confirm,  a  continuous 
stream  of  air  may  be  passed  through  the  solution  into  a  metal 
vessel  containing  an  insulated  negative  electrode,  the  vessel 
itself  being  connected  with  the  positive  pole  of  the  battery. 
The  voltage  is  not  important,  but  may  be  from  50  to  300  volts, 
according  to  the  size  of  the  vessel.  The  rate  of  decay  of  the 
imparted  activity  on  the  negative  electrode  may  then  be  deter- 
mined, commencing  after  the  lapse  of  five  or  six  hours  from 
the  time  it  is  removed  from  the  vessel.  Tn  this  time  any 
radium  or  actinium-imparted  activity  will  have  completely 
decayed,  whereas  the  thorium-imparted  activity  decays  to  half 


144  RADIO-ACTIVITY. 

the  initial  value  in  11  hours,  and  should  still  be  noticeable 
after  two  days. 

To  test  for  the  presence  of  actinium,  the  air  is  blown  out 
from  the  solution  as  before,  and  then  a  rapid  current  is  blown 
directly  into  the  electroscope  for  some  two  or  three  minutes. 
While  the  air-stream  is  passing  the  electroscope  should  leak 
rapidly.  The  rate  of  decay  of  the  actinium  emanation  is  so 
rapid  that  it  cannot  be  well  measured  by  this  rough  means, 
but  its  presence  may  be  confirmed  by  continuing  the  air  stream 
for  some  time,  when  there  should  be  left  in  the  electroscope  an 
imparted  activity  very  similar  to  that  of  radium  in  its  rate  of 
decay,  but  slightly  slower.  If  care  is  taken  to  completely 
blow  out  all  the  accumulated  radium  emanation  from  the 
solution  before  the  air-stream  is  sent  into  the  electroscope, 
the  possibility  of  the  radium-imparted  activity  is  excluded, 
and  the  presence  of  actinium  may  be  inferred. 

If  in  these  tests  an  emanation  is  obtained  which  differs 
essentially  from  those  above  described,  the  existence  of  a  new 
radio-element  may  be  inferred.  The  characteristics  of  its 
radio-activity  may  be  worked  out  and  the  element  sufficiently 
defined  before  a  single  chemical  separation  of  the  mineral  is 
effected. 

The  principles  underlying  these  methods  of  radio-active 
analysis  will  be  readily  understood  from  previous  chapters. 
The  most  important  is  that  the  radio-activity  measured  is 
the  product  of  the  radio-active  constant  A  and  the  number 
of  atoms  present  of  the  type  of  matter  in  question.  If  A  is 
relatively  small,  as  in  the  case  of  the  radium  emanation,  AN 
is  small  when  N  is  considerable.  Hence,  although  the  number 
of  atoms  of  radium  emanation  present  after  the  lapse  of  five 
minutes'  accumulation  may  be  as  great  as  the  number  of 
thorium  emanation  atoms,  only  the  latter  exhibits  a  detectable 
radio-activity.  The  more  slowly  a  transition-form  loses  its 
activity  the  longer  must  it  be  allowed  to  re-accumulate  in 
order  for  it  to  be  detected  by  its  activity.  Similarly,  only  the 
first  type  of  the  radium-imparted  activity  matter  is  experi- 
mentally observed  when  the  emanation  is  allowed  to  remain  in 
the  electroscope  only  a  minute  or  so,  and  this,  after  the  latter 
is  blown  out,  decays  practically  to  zero  in  five  or  six  minutes. 
If  left  in  half  an  hour  or  more,  the  emanation  produces  sufficient 


RADIO-ACTIVE  PROPERTIES  OF  RADIUM.  145 

of  the  slower-changing  third  type  to  be  quite  apparent 
after  the  rapidly-changing  first  type  has  decayed.  These 
examples  might  be  multiplied  indefinitely.  Extended  to  the 
radio-active  minerals  themselves  over  a  period  of  years  or 
centuries,  they  throw  light  on  the  fundamental  problem,  with 
which  this  chapter  opened,  of  the  present  existence  of  so 
powerful  a  radio-active  element  as  radium. 


146 


RADIO-ACTIVITY. 


APPENDIX. 


Two  tables  are  appended  which  summarise  the  present 
knowledge  of  the  course  of  atomic  disintegration.  In  the  first, 
the  complete  series  of  disintegrations  in  the  case  of  uranium, 

Table  I. 


UKANIUM. 

THOEIUM. 

RADIUM. 

/\ 

/\ 

/\ 

X       X 

^        ^ 

¥- 

X 

U  ranium  X  o  particles 

Thorium  X    a  particles 

Emanation    a  particles 

P\^ 

r\^ 

of  radium 

?             8  particles 

^^ 

Y 
Emanation   a  particles 

i 

^ 

of  thorium       (perhaps 

Matter        \  o  particles 

—  '  .                                                      

ACTINIUM. 

|8  also) 

causing  the  1 
imparted 

/\ 

^^^-^^^ 

activity  of 

}£.        ^ 

"^-  ,% 

radium. 

a  particles 
Emanation  I.       (?) 

Y 
Matter        \  a  particles 
causing  the 

First  type  , 

"^-^^^ 

imparted     1 

y 

^-^ 

o  particles 

activity  of   f 
thorium. 
First  type  J 

1 

Ditto.      j 
Second     I 
type       ) 

a  particles 

Emanation  II.  (?) 

1 

1 
1 

Ditto. 

Y 

Matter 

Second  type 

Ditto.       ) 
Third       V 

causing 

^""^\^ 

type     J 

imparted 

i        •      ""^^ 

activity 

?               a  and  /3 

I 

^ 

r 

particles 

Y 
Matter          ^ 

aand/S 

4.        >>* 

causing  the 

particles 

?                a  and  8 

"induced 

particles 

activity  of 
radium 

. 

with  slow 

rate  of 

dissipation" 

(?  polonium]- 

Y 

-^ 

9 

o  particles 

APPENDIX. 


147 


thorium,  radium  and  actinium  have  been  tabulated.  In  the 
second,  the  unstable  elements  have  been  arranged  in  the  order 
of  their  instability  together  with  their  radio-active  constants, 
A,  or  the  fraction  changing  per  second,  and  the  period  of  their 
average  lives,  as  given  by  I/ A..  The  latter  is  the  time  required 
for  the  quantity  to  be  reduced  to  l/e  (0'368)  of  the  initial, 
which  is  1-45  times  longer  than  that  required  for  the  quantity 
to  be  reduced  to  half.  The  values  of  A  for  the  last  three 
examples  are  deduced  in  Chapter  XL 


Table  II. 


Element. 

A 

I/A 

Actinium  emanation  ...         ... 

1-7x10-* 

5*7  seconds 

Thorium  emanation    ...         ...i 

1-16  xlO~2 

87       „ 

Eadium-imparted  activity  I.  ... 

4  x  10~3 

4  minutes 

Ditto           ditto         II.... 

5-4  x  10-4 

30       „ 

Ditto           ditto        III.... 

4-1  x  10-4 

41       ,, 

Actinium  imparted  activity    .  .  .  j 

SxlO-4 

58       „ 

Thorium-imparted  activity  I.  .  .  .  i 

r 

! 

Ditto           ditto         II.... 

l-7xlO-5 

IGhrs.  30min. 

Radium  emanation      ...         ...| 

2-2  x  10-° 

5  days  8  hours 

Thorium  X      ...          ...          ...j 

2X10-6 

5  days  19  hours 

Uranium  X      i 

3  6  x  10-7 

32  days 

Polonium         about 

2  x  10-8 

about  1  6  months 

Actinium          ...          ...         ... 

! 

1 

Radium            ...         ...         ...J 

3  x  10-11 

1,150  years 

Uranium          ...           )       , 
Thorium                        /     abont 

3  x  10-17 

about  109  years 

CHAPTEK  X. 


THE  MATERIAL  PROPERTIES  OF  THE  RADIUM 
EMANATION  AND  ITS  TRANSMUTATION  INTO 
HELIUM. 

Material  Properties  of  Radio-active  Matter. — Condensation  of  thi  Emana- 
tions by  Liquid  Air. — Differences  of  Behaviour  between  the  Thorium 
and  Radium  Emanations. — Volatility  of  Matter  causing  Imparted 
Activity. — Diffusion  Coefficient  of  Radium  Emanation. — Production 
of  Helium  from  Radium. — Production  from  the  Emanation. — Phos- 
phorescent Spectrum  of  Radium  Compounds. —  Volume  occupied  by 
Radium  Emanation. — Diminution  on  Keeping. — Possibility  that  the 
a-Particle  is  a  Helium  Atom. — Law  of  the  Conservation  of  Mass  need 
not  apply  to  Sub-atomic  Changes. 

Radio-activity  is  the  property  of  but  a  few  atoms  only  of 
the  total  mass  in  any  given  instant.  A  certain  fraction, 
represented  by  A,  contributes  the  activity  in  any  given  second, 
while  the  vast  preponderance,  1  -  A,  is,  strictly  speaking,  not 
radio-active  at  all.  It  is,  for  the  time  being,  ordinary  matter 
which  must  possess  a  normal  chemical  and  physical  nature  in 
addition  to  and  independent  of  the  radio-active  properties 
contributed  by  the  fraction  undergoing  disintegration.  Each 
transition-form  is  a  new  element,  although  an  unstable  one, 
and  some  information  about  its  ordinary  material  properties 
may  be  obtained.  An  example  may  make  this  clear.  The 
radium  emanation  passes  through  tubes  containing  various 
reagents  which  would  completely  absorb  all  the  known  gases 
except  the  members  of  the  argon  family.  Like  the  latter,  it 
may  be  sparked  over  potash  in  an  atmosphere  of  oxygen  or 
kept  in  contact  with  red-hot  magnesium  and  calcium  for 
many  hours  without  being  appreciably  absorbed.  The  con- 
clusion is  indicated  that  the  emanation  is  a  chemically 


150  RA  DIG- A  GTIVIT  Y. 

inert  element  of  the  argon  family.  In  this  and  similar  work 
the  chemical  nature  of  the  matter  is  the  real  object  of  study, 
the  radio-activity  merely  furnishing  the  means  of  detection 
and  measurement.  The  special  interest  about  this  work  is 
the  smallness  of  the  quantity  of  material  employed.  Con- 
sider, for  example,  the  case  of  the  phenomenon  exhibited  by 
the  emanations  of  being  condensed  at  low  temperatures.  If 
the  volatility  or  vapour  pressure  of  an  ordinary  gas  were 
investigated  by  direct  methods,  the  limit  of  detection  would  be 
reached  with  pressures  of  T^mm.  of  mercury.  In  the  case 
of  the  emanations  we  can  study  accurately  the  effect  of 
changing  temperature  on  the  volatility  of  quantities  a  million 
million  times  less,  and  the  results,  therefore,  are  of  great 
general  interest  from  a  purely  physical  point  of  view,  apart 
altogether  from  the  special  means  by  which  the  result  is 
obtained. 

Condensation  of  the  Radio-active  Emanations. — This  property,  in 
the  case  of  the  radium  emanation,  may  be  made  the  subject  of 
a  striking  and  beautiful  lecture  experiment.  If  the  emanation, 
obtained  by  heating  or  dissolving  a  few  milligrammes  of  pure 
radium  bromide,  or  by  storing  a  solution  of  the  latter  in  a 
closed  bottle  for  a  week  since  the  emanation  was  last  removed, 
is  swept  by  a  slow  stream  of  dry  air  into  a  glass  spiral  tube 
immersed  in  liquid  air,  practically  the  whole  of  the  emanation 
condenses  at  the  point  where  the  spiral  enters  the  liquid 
air,  causing  the  glass  to  nuoresce  brilliantly.  The  apparatus 
usually  employed  by  the  Author  is  shown  in  Fig.  32.  Some 
hours  before  the  lecture  the  emanation  from  the  radium  is 
swept  into  A  and  the  taps  closed.  A  is  a  glass  tube  in  which 
has  been  sealed  a  piece  of  willemite.  It  has  been  mentioned 
that  the  /2-rays  of  radium  are  mainly  operative  in  causing  this 
mineral  to  nuoresce,  so  that  the  luminosity  of  the  mineral 
increases  for  some  hours  after  the  emanation  is  introduced  into 
the  tube,  and  after  five  or  six  hours  is  mainly  due  to  the  imparted 
activity,  and  not  to  the  emanation.  Kunzite  would  show  this 
effect  even  better,  but  is  less  brilliantly  luminous.  For 
showing  the  condensation  of  the  emanation  the  tube  is  attached 
to  the  glass  spiral  B  immersed  in  a  vessel  containing  liquid 
air  C.  To  the  other  end  of  the  spiral  a  long  glass  tube  D  is 
attached.  In  D  is  placed  a  long  narrow  strip  of  mica  painted 


PROPERTIES  OF  THE  RADIUM  EMANATION.       151 

with  phosphorescent  zinc  sulphide.  The  easiest  way  to  coat  the 
mica  is  to  cover  it  with  a  hot  film  of  gelatine  solution,  and  sieve 
on  the  zinc  sulphide  after  the  gelatine  has  partly  set.  With  this 
apparatus,  provided  that  an  extremely  slow  air-stream  is  used, 
the  whole  of  the  emanation  is  condensed  at  a  point  in  the 
spiral.  The  tube  A  may  then  be  removed  and  the  air-stream 
sent  into  B  direct.  On  removing  C  the  stage  at  which  the 
emanation  volatilises  may  be  clearly  seen  by  the  passage  of 
the  glowing  gas  along  the  spiral  into  D.  Then  the  gradual 


FIG.  32. — Lecture  Experiment  for  Showing  the  Condensation  of  the 
Badium  Emanation  by  Liquid  Air. 


ascent  of  the  emanation  as  a  zone  of  intense  luminosity  may 
be  watched  in  D  as  the  air-stream  is  continued.  The  lower 
parts  of  the  screen  in  D,  the  whole  spiral,  and  also  the 
willemite  in  A,  continue  to  glow,  owing  to  the  imparted 
activity,  after  the  emanation  has  passed  on,  but  this  glow 
completely  disappears  in  the  course  of  a  few  hours.  To 
illustrate  the  properties  of  the  emanation  a  long  narrow  tube 
may  be  inserted  between  A  and  B  before  the  condensation 


152 


RADIO-ACTIVITY. 


is  carried  out.  This  tube  might  be  made  to  pass  round  the 
lecture  table,  and  if  small  diamonds  are  introduced  at  different 
points  the  arrival  of  the  emanation  at  the  various  points  could 
be  rendered  visible. 

The  apparatus  used  in  the  detailed  investigation  of  the  con- 
densation of  the  emanations  (Rutherford  and  Soddy,  Phil.  Mag., 
1903,  VI.,  5,  p.  561)  is  shown  in  Fig.  33.  The  emanation  was 
carried  by  means  of  a  steady  stream  of  some  suitable  gas 


tun 


FIG.  33. 

through  the  doubly-wound  copper  spiral,  which  was  immersed 
in  a  bath  of  liquid,  kept  well  stirred,  at  the  temperature 
required.  The  liquid  employed  was  ethylene,  which  boils  at 
-  103°C.  and  freezes  at  -  169°C,  The  outside  tube  was  cooled 
with  liquid  air,  and  ethylene  gas  passed  in  through  A  until 
the  level  of  the  liquefied  ethylene  was  well  above  the  top  of 
the  spiral.  The  emanation  was  passed  through  the  spiral  after 
the  bath  had  been  cooled  by  means  of  liquid  air  slightly  below 


PROPERTIES  OF  THE  RADIUM  EMANATION.       153 

the  temperature  of  condensation.  The  apparatus  was  wrapped 
round  with  felt  and  allowed  to  warm  up  slowly,  and  the 
temperature  was  noted  at  which  the  emanation  volatilised  and 
was  swept  out  of  the  spiral  by  the  gas  stream. 

The  general  arrangement  of  the  apparatus  for  the  case  of 
the  radium  emanation  is  shown  in  Fig.  34.  The  emanation 
was  stored  in  the  gas  holder  B,  and  small  quantities  could  be 
sent  as  required  into  the  gas  stream  kept  passing  continually 
into  the  spiral  from  A.  The  temperature  of  the  bath  was 
known  by  the  electrical  resistance  of  the  copper  spiral,  and 
this  was  determined  by  keeping  a  constant  current  of  about 
an  ampere  flowing  through  it,  and  measuring  the  difference  of 

IKlillivoltmeter  Ammeter 

Res. 


To  Earth 


FIG.  34. 


potential  between  two  points  at  the  top  and  bottom  of  the 
spiral  by  means  of  a  milli-voltmeter.  In  Fig.  35  the  resistance 
of  the  copper  spiral  is  plotted  on  the  vertical  axis,  and  the 
temperatures  in  degrees  Centigrade  on  the  horizontal  axis. 
The  fixed  points  chosen  in  the  calibration  of  the  spiral  were 
the  boiling  and  freezing  points  of  water,  the  boiling  and 
freezing  points  of  ethylene,  and  the  boiling  point  of  liquid  air 
of  known  composition.  The  temperatures  of  the  condensation 
of  the  thorium  emanation  (-120°C.)  and  of  the  radium 
emanation  (  —  150°C.)  were  found  by  interpolation.  With  the 
radium  emanation  the  point  of  volatilisation  and  condensation 
is  very  sharp.  In  one  experiment  at  -  154°C.  no  emanation 


154 


RADIO-ACTIVITY. 


was  observed  in  the  gas  stream,  although  less  than  y^i 
part  could  have  been  detected,  while  at  —  150°C.  the  greater 
part  had  volatilised.  The  decay  of  the  activity  of  the  thorium 
emanation  is  so  rapid  that  the  emanating  compound  has  to 
be  retained  continuously  in  the  gas  stream.  It  is  then  found 
that  some  emanation  escapes  condensation  when  the  tempera- 
ture is  as  low  as  -  150°C.,  but  the  greater  part  is  condensed 
at  much  higher  temperatures.  The  point  given  above, 
—  120°C.,  represents  the  temperature  at  which  condensation 
commences  to  take  place.  In  the  case  of  the  radium  emana- 
tion, no  great  difference  is  observed  between  the  temperature 
of  initial  volatilisation  and  complete  condensation,  provided 
sufficiently  slow  streams  of  gas  are  employed. 


Per  cent.  Resistance  of  Copper  Spiral. 

H-itOC£I^Uia~400COC 
0  OOOOOOOOOC 

X 

s 

X 

1.  Pt.  Eth 

ylene. 

> 

iCondens.  Pt.  Tho 

r.  Eman. 

ondens.  F 

>t.  Rad.  E 
Ptr-Ethy 

Soil.  Pt.  L 

man. 
Icne;  — 
iquid  Air 

— 

S^Freez 
JVJ 

1 

^x 

VVN 

Nx 

-40          -80         -120       -160        -200        -240       -280 
Temperature  (Cent.) 
FIG.  35. 

This  difference  between  the  two  emanations  was  further 
investigated  by  means  of  the  apparatus  shown  in  Fig.  36, 
which  is  arranged  for  the  case  of  the  thorium  emanation.  The 
gasholder  B  and  T-piece  of  Fig.  34  replaces  the  thorium  oxide 
tube  A  for  experiments  with  the  radium  emanation.  Instead 
of  the  emanation  being  carried  by  a  steady  current  of  gas  it  is 
transferred  to  the  spiral,  and  thence  to  the  testing  vessel,  by 
means  of  the  mercury  pump  P.  This  was  effected  by  raising 
or  lowering  the  mercury  in  the  pump  and  by  opening  and 
closing  the  various  taps  as  required.  In  this  way  the  experi- 
ments could  be  performed  in  an  atmosphere  of  any  gas  at  any 


PROPERTIES  OF  THE  RADIUM  EMANATION.       155 

pressure  below  atmospheric,  and  the  emanation  allowed  to 
remain  in  the  condensing  spiral  any  definite  time  before  being 
removed.  It  was  found  that  the  temperature  of  condensation 
in  the  case  of  the  radium  emanation  was  substantially  the 
same  however  the  experiment  was  performed.  But  for  the 
thorium  emanation  the  proportion  of  the  total  condensing 
at  any  temperature  varied  (1)  with  the  nature  of  the  gas 
used,  a  greater  proportion  condensing  in  an  atmosphere  of 
hydrogen  than  of  oxygen ;  (2)  with  the  time  allowed,  a  greater 
proportion  condensing  the  longer  the  emanation  was  allowed 
to  lemain  in  the  spiral.  The  proportion  escaping  condensa- 


Ammeter  Millivoltmeter 


FIG.  36. 

tion  was  always  greater  when  a  current  of  air  was  kept  passing 
than  when  the  experiments  were  performed  by  means  of  the 
pump  in  a  stationary  atmosphere.  The  temperature  of  initial 
condensation,  -  120°C.,  was,  however,  always  substantially  the 
same  whatever  the  conditions. 

The  difference  between  the  behaviour  of  the  thorium  and 
the  radium  emanation  is  capable  of  a  simple  explanation  on 
the  disintegration  theory.  Consider  quantities  of  the  two 
emanations  possessing  similar  radio-activity— i.e.,  giving  a 
similar  number  of  a -rays  per  second.  The  radio-activity  is 
proportional  to  the  number  of  atoms  disintegrating  per 


156  RADIO- ACTIVITY. 

second — i.e.,  to  AN,  where  N  is  the  total  number  present  and 
A  the  radio-active  constant.  Since  A  for  the  radium  emanation 
is  6,000  times  smaller  than  for  the  thorium  emanation,  it 
follows  that,  when  AN  is  equal  for  the  two  cases,  the 
number  of  atoms  of  emanation  present  must  be  6,000  times 
greater  in  the  case  of  the  radium  emanation  than  in  the 
case  of  the  thorium  emanation.  Condensation  can  only 
take  place  by  the  approach  of  two  or  more  of  the 
condensing  molecules  into  each  other's  sphere  of  influence, 
and  it  is  to  be  expected,  when  the  quantities  present  are 
extremely  small,  that  the  amount  condensing  will  depend 
on  the  concentration  of  the  particles,  the  time  allowed  for  the 
process  to  take  place,  and  the  nature  of  the  atmosphere  in 
which  the  experiments  are  performed.  The  concentration  of 
the  particles  would  be  less  when  the  emanation  was  carried  by 
a  steady  stream  of  gas  than  in  a  stationary  atmosphere.  The 
particles  would  diffuse  more  rapidly  in  hydrogen  than  in 
oxygen,  and  the  rate  of  condensation  would  be  accelerated. 
This  effect  of  time  on  the  progress  of  condensation  would 
rapidly  diminish  as  the  number  of  condensing  particles  were 
increased.  It  would  probably  diminish  as  the  square  of  the 
number  present.  Hence,  if  a  similar  number  of  atoms 
remained  uncondensed  in  comparable  experiments  with  the 
radium  and  thorium  emanations,  the  radio-activity  of  the 
former  might  be  well  below  the  limit  of  ordinary  detection 
when  the  radio-activity  of  the  latter  was  large.  To  obtain 
comparable  results  with  the  two  emanations  the  measuring 
instruments  should  be  made  6,000  times  more  sensitive  in  the 
case  of  that  of  radium  than  for  that  of  thorium.  The 
ionisation  current  in  the  experiments  on  the  thorium  emanation 
was  probably  about  10~10  amperes.  An  ionisation  current  of 
10~u  amperes  is  quite  within  the  experimental  range,  and  it 
is,  therefore,  quite  possible  to  subject  this  explanation  to  direct 
experimental  test.  This  has  not  yet  been  done. 

From  considerations  to  be  deduced  in  the  next  chapter, 
it  is  probable  that  the  thorium  emanation  obtainable  from 
10  grammes  of  thorium  oxide  if  confined  in  1  cubic  cm.  volume 
would  only  exert  a  pressure  of  between  10~16  and  10~17  atmo- 
sphere, so  that  the  emanation  obtainable  from  a  million  tons  of 
thorium  oxide,  if  concentrated  in  this  volume,  would  only  just 


PROPERTIES   OF  THE  RADIUM  EMANATION.        157 

produce  a  pressure  that  could  be  directly  measured.  It  is, 
therefore,  matter  for  remark  that  the  emanation  can  be  made 
to  condense  completely  at  any  temperature,  and  the  question 
remains,  perhaps,  somewhat  doubtful  as  to  whether  the  pheno- 
menon is  really  one  of  simple  gaseous  condensation  at  all.  It 
has  been  suggested  that  the  gas  used  to  carry  the  emanation 
condenses  on  the  walls  of  the  spiral  by  the  action  of  capillary 
forces  as  a  surface  film,  and  carries  with  it  the  particles  of 
emanation  by  attraction,  possibly  of  a  chemical  nature.  Such 
a  surface  condensation  would  presumably  only  occur  in  a  gas 
below  its  critical  temperature,  and  should,  therefore,  be  absent 
in  perfectly  pure  hydrogen  or  helium.  It  would  be  interest- 
ing to  repeat  the  experiments  in  a  stream  of  hydrogen  of  the 
highest  possible  purity.  It  is  possible  that  condensation  in 
these  circumstances  might  not  occur. 

In  this  connection  may  be  mentioned  the  work  of  Miss 
Gates  (Physical  Review,  1903,  16,  p.  300),  who  found  that  the 
matter  causing  the  imparted  activity  of  thorium  and  radium 
could  be  volatilised  at  temperatures  above  a  red  heat  and  be 
deposited  on  neighbouring  cold  surfaces.  This  indicates  that 
phenomena  similar  to  the  condensation  and  volatilisation  of 
the  emanations  take  place  with  the  imparted  activity  matter 
at  high  temperatures. 

Another  property  of  the  emanation  of  radium  which  has 
been  investigated  is  its  density,  as  shown  by  its  coefficient 
of  diffusion  into  air.  The  first  experiments  were  performed 
by  Rutherford  and  Miss  Brooks.  They  determined  the  rate 
at  which  the  emanation  confined  in  one  half  of  a  long  cylinder 
diffused  into  the  other  half  when  a  partition  between  the 
two  halves  was  withdrawn.  They  found  a  value  for  the 
coefficient  of  diffusion  of  about  0'08,  which  is  comparable  with 
that  of  ethyl  ether,  and  indicates  a  density  in  the  neigh- 
bourhood of  80  (H  =  l).  If  the  molecule  of  the  emanation  is 
assumed  to  be  monatomic,  like  argon,  the  atomic  weight  is, 
therefore,  in  the  neighbourhood  of  180. 

The  great  difficulty  in  the  experiments  is  to  eliminate  the 
effect  of  the  imparted  activity  produced  from  the  emanation 
from  that  of  the  latter  by  itself,  and  the  rate  of  diffusion  tends 
in  consequence  to  appear  lower  than  the  true  value.  Owing 
to  the  very  rapid  initial  changes  in  the  radio-activity  of  the 


158  RADIO-ACTIVITY. 

matter  causing  the  imparted  activity  it  is  difficult  to  correct 
accurately  for  this  error.  The  value  given  above,  0-08, 
is  20  per  cent,  greater  than  the  unconnected  value.  A  deter- 
mination of  the  coefficient  has  also,  been  made  by  Curie  and 
Danne  (Comptes  Rendus,  1903,  136,  p.  1,314),  who  obtained  the 
value  O'l,  which  indicates  a  density  nearer  to  that  of  alcohol 
(46)  than  of  ether.  Most  recently  Bnmstead  and  Wheeler 
(Am.  Journ  of  Science,  February,  1904,  p.  97)  obtained  a  value 
indicating  a  density  of  180,  so  that  the  evidence  is  conflicting, 
probably  for  the  reason  already  given.  The  latter  have 
definitely  proved  that  the  radio-active  gas  obtained  from  the 
tap-water  at  New  Haven,  U.S.A.,  and  from  the  soil  in  the  same 
neighbourhood,  is  the  radium  emanation,  for  the  rate  of  decay 
of  its  activity  and  its  rate  of  diffusion  are  similar  to  the  latter. 
The  existence  of  the  radio-active  gas  in  tap-water  was  first 
shown  by  J.  J.  Thomson  in  the  Cambridge  water,  and 
experiments  by  H.  S.  Allen  on  the  rate  of  decay  of  its  activity 
indicated  that  it  was  the  same  as  the  radium  emanation.  The 
existence  of  this  emanation  in  the  soil  was  first  noticed  by 
Elster  and  Geitel. 

The  Production  of  Helium  from  Radium. — (Eamsay  and 
Soddy,  Proc.  Roy.  Soc.,  Vol.  LXXIL,  1903,  p.  204.)  The 
intense  radio-activity  of  pure  radium  compounds,  and  the 
relatively  rapid  rate,  therefore,  at  which  it  must  be  supposed 
to  be  disintergrating,  made  it  appear  likely  that  if  helium  were 
one  of  the  ultimate  products  formed,  as  has  already  been 
suggested  (p.  117),  it  might  be  possible  to  detect  it  by  direct 
spectroscopic  methods  if  a  reasonable  time  were  allowed  for  it 
to  accumulate.  The  apparatus  used  is  shown  in  Fig.  37.  In 
two  separate  experiments  20  and  30  mgs.  of  pure  radium 
bromide,  which  had  been  kept  for  some  months  in  the  dry 
solid  state,  were  employed.  From  the  general  analogy  of 
helium  to  the  radium  emanation,  both  in  chemical  nature  and 
the  phenomenon  they  exhibit  of  being  occluded  and  of  being 
liberated  by  heat  and  solution,  it  was  assumed  that  the  helium 
if  produced  would  not  escape  from  the  radium  compound  until 
it  was  heated  or  dissolved.  The  radium  bromide  was  intro- 
duced into  the  bulb  A  which  was  then  sealed  at  the  clotted 
line  M  to  the  bulb  B,  carrying  two  taps.  This  was  then  com- 
pletely exhausted,  the  lower  tap  closed,  and  water,  boiled  free 


PROPERTIES   OF  THE  RADIUM  EMANATION.       159 

from  air,  allowed  to  flow  into  B.  The  seal  at  the  dotted  line 
N  was  then  made.  C  is  a  tube  in  which  is  a  thin  copper 
spiral  wire  which  can  be  heated  by  an  electric  current.  D  is 
a  phosphorous  pentoxide  tube  to  absorb  moisture.  E  is  a 
capillary  U  tube  which  is  cooled  in  liquid  air  during  the 
experiment.  The  emanation  and  any  C0.2  present  are  con- 
densed here  and  prevented  from  entering  the  spectrum  tube. 


To  Pump 


FIG.  37. 

The  tap  L  is  connected  to  the  mercury  pump  not  shown.  F 
is  the  spectrum  tube  shown  half  full-size  at  H.  The  copper 
spiral  is  first  partially  oxidised  by  filling  the  tube  with  oxygen 
from  the  burette  G  and  keeping  the  spiral  at  dull  redness  by  a 
current.  The  whole  apparatus  is  thoroughly  exhausted  and 
all  taps  closed.  Water  is  now  admitted  from  B  into  A,  the 


160  RADIO-ACTIVITY. 

radium  bromide  dissolves  and  gives  up  its  occluded  gases, 
which  are  admitted  into  C.  The  red-hot  spiral  absorbs  the 
hydrogen  and  oxygen,  and  the  water  produced  is  absorbed  in 
D.  Mercury  is  now  allowed  to  flow  up  from  G,  and  the  gases 
forced  into  the  spectrum  tube  through  the  cooled  U  tube. 
The  spectrum  on  examination  proved  to  be  practically  the 
complete  spectrum  of  helium. 

In  the  next  experiments  the  emanation  from  50  mgs, 
of  radium  bromide  (which  had  been  kept  in  solution  in  a 
closed  vessel  until  practically  the  equilibrium  amount  of 
emanation  had  accumulated)  was  condensed  in  a  spectrum 
tube,  and  all  volatile  gases,  including  any  helium,  if  present, 
were  removed  by  the  pump.  The  tube  was  then  sealed  off. 
The  yellow  line  of  helium  appeared  after  it  had  been  kept 
three  days,  and  in  five  days  the  complete  helium  spectrum 
was  obtained.  This  proved  conclusively  that  helium  is  being 
produced  from  the  emanation.  As  the  latter  changes  into 
helium  in  the  spectrum  tube,  a  fresh  crop  of  emanation  is 
being  produced  by  the  radium.  The  radium  changes  into 
helium  via  the  emanation.  The  amount  of  helium  obtained  in 
these  experiments  was  estimated  at  between  10'4  and  10~5 
cubic  cm. 

Curie  and  Dewar  (Chem.  News,  February  19,  1904)  examined 
the  gas  evolved  from  400  mgs.  of  radium  bromide  on  fusion 
in  a  quartz  bulb,  but  did  not  detect  the  helium.  They 
exhausted  the  bulb,  sealed  it  and  sent  it  to  Deslandres  for 
spectroscopic  examination.  The  latter  photographed  the 
spectrum  obtained  by  passing  a  discharge  through  the  bulb 
after  it  had  been  sealed  some  weeks,  and  found  it  to  be  that 
of  pure  helium. 

Sir  William  and  Lady  Huggins  (Proc.  Roy.  Soc.,  1903,  72, 
pp.  196  and  409)  have  examined  the  light  emitted  by  com- 
pounds of  radium,  and  have  detected  bands  in  the  ultra-violet 
corresponding  to  the  negative  glow  spectrum  of  nitrogen. 
Crookes  and  Dewar,  according  to  a  statement  made  by  the 
latter  at  the  British  Association  Meeting,  1903,  have  shown 
that  these  bands  are  not  produced  unless  nitrogen  is  present 
in  the  atmosphere  to  which  the  radium  is  exposed.  In  vacuo, 
and  also  in  pure  helium,  the  spectrum  of  the  luminosity  is 
continuous  and  no  bands  are  present. 


PROPERTIES  OF  THE  RADIUM  EMANATION.        161 

Volume  of  the  Radium  Emanation. — Continuing  the  research 
on  the  production  of  helium  from  radium,  Ramsay  and  Soddy 
have  made  a  determination  of  the  volume  occupied  by  the 
emanation  produced  from  a  known  quantity  of  radium  in  a 
known  time.  This  is  even  less  than  the  volume  of  the  helium 
produced,  for  two  reasons.  First,  the  emanation  is  much 
denser,  and,  secondly,  the  quantity  does  not  continuously 
accumulate,  but  attains  an  equilibrium  value  when  the  rate  of 
production  equals  the  rate  of  disintegration.  Since  (p.  131) 


FIG.  38. 


#o=A.No,  the  equilibrium  quantity  N0  is  the  quantity  produced 
per  second,  q0,  divided  by  the  radio-active  constant.  Or,  since 
I/A  represents  the  average  life  of  the  atom,  the  equilibrium 
quantity  is  the  amount  produced  in  a  period  equal  to  the 
average  life,  which  for  the  radium  emanation  is  5 '3  days. 

The  apparatus  employed  is  shown  in  Fig.  38.  The 
hydrogen,  oxygen  and  emanation  that  have  accumulated  a 
known  time  in  a  previously  exhausted  bulb  containing  the 


162  RADIO-ACTIVITY. 

radium  solution  are  introduced  into  E  and  exploded.  A 
small  excess  of  hydrogen  always  remains.  In  E  a  lump  of 
solid  caustic  potash  floats  on  the  surface,  of  the  mercury  and 
absorbs  carbon  dioxide.  0  is  a  phosphorus  pentoxide  tube, 
B  is  a  small  bulb  that  can  be  cooled  externally  by  liquid  air, 
and  A  is  a  piece  of  fine  thermometer  tubing  sealed  at  the 
upper  end.  The  whole  is  carefully  exhausted,  the  pump 
connection  closed,  and  the  hydrogen  and  emanation  admitted 
into  the  cooled  bulb  C.  The  hydrogen  is  then  pumped  out 
completely  and  mercury  allowed  to  flow  up  from  E  until  it 
cuts  off  the  pump-connection.  B  now  contains  only  the 
condensed  emanation  freed  from  all  other  gases.  The  liquid 
air  is  removed,  the  emanation  allowed  to  volatilise,  and  mercury 
allowed  to  fill  the  bulb  and  compress  the  emanation  into  A. 
At  atmospheric  pressure  the  volume  occupied  was  found  to  be 
excessively  small,  and  it  was  measured  under  considerably 
reduced  pressure.  The  equilibrium  quantity  of  emanation  from 
60  mgs.  of  radium  bromide  was  found  to  occupy  between 
0-03  and  0-04  cubic  mm.  at  0°C.  and  760mm.  The  emanation 
produced  in  5 '3  days  by  1  gramme  of  radium  (element)  there- 
fore occupies  a  volume  of  about  1  '3  cubic  mm. 

The  behaviour  of  the  tiny  bubble  of  gas  was  investigated 
from  day  to  day.  In  one  experiment  the  volume  steadily 
diminished  with  time,  roughly  at  the  same  rate  and  according 
to  the  same  law  as  the  activity  decays.  In  about  three  weeks 
the  volume  had  shrunk  practically  to  zero,  being  less  than 
1  per  cent,  of  that  initially  occupied. 

The  following  table  shows  the  decrease  of  volume  measured 
in  this  experiment : — 

Volume.  Cubic  mm. 

After  1st  day        0-027 

„     3rd  day        0011 

„     6th  day        0-0063 

„     9th  day  0'0041 

Final  volume         0*0004 

This  uniform  decrease  of  volume  is,  however,  not  always 
observed.  In  some  experiments  the  volume  increased,  but 
whether  this  is  due  to  surface  films  of  gas  on  the  glass  or 
other  disturbing  cause  or  is  a  real  effect  has  not  been  settled. 


PROPERTIES   OF  THE  RADIUM  EMANATION.        163 

Rutherford  has  suggested  that  the  a  particle  is  an  atom  of 
helium.  The  mass  of  the  particle,  according  to  his  prelimi- 
nary determinations,  is  1-6  times  that  of  the  hydrogen  atom, 
whereas  the  mass  of  the  helium  atom  in  terms  of  hydrogen  is 
4.  It  seems  likely  that  this  discrepancy  may  be  accounted 
for,  and  that  the  a  particle  will  ultimately  be  shown  to  be  a 
helium  atom.  It  seems  certain  that  the  a  particles  of  all  the 
radio-active  bodies  are  likely  to  be  of  the  same  mass,  and  that 
the  comparatively  narrow  limits  in  which  they  differ  in  pene- 
trating power,  &c.,  result  from  their  possessing  different 
velocities.  As  Strutt  has  pointed  out  in  a  recent  Paper 
(Proc.  Roy.  Soc.  1904,  73,  p.  191),  helium  is  very  abundant 
in  minerals  like  monazite,  which  contain  large  quantities  of 
thorium,  but  very  little  radium.  This  hypothesis  would, 
therefore,  lead  to  the  belief  that  helium  is  a  common  product 
in  the  disintegration  of  all  the  radio-elements. 

The  diminution  of  the  volume  of  the  emanation  to  zero  is 
well  explained  on  this  view,  for,  if  the  helium  is  produced 
from  the  emanation  in  the  form  of  projected  a  particles,  it  is 
to  be  expected  that  the  latter  will  bury  themselves  in  the 
glass,  owing  to  their  enormous  velocity.  The  experiment  was, 
therefore,  performed  of  "  washing  "  out  the  thermometer  tube 
with  oxygen  after  the  emanation  had  spent  itself,  re-exhausting 
and  heating  the  glass.  On  passing  a  discharge  through  the 
tube,  by  means  of  external  tin-foil  caps  attached  to  the  ends, 
the  helium  spectrum  was  clearly  seen,  but  the  tube  punctured 
before  it  could  be  confirmed.  The  experiments  are  difficult, 
and  the  results,  in  so  far  as  they  bear  on  the  question  as  to 
whether  the  a  particle  is  an  atom  of  helium,  must  be  considered 
only  provisional  at  the  present  stage. 

It  may  be  well  to  define  the  limits  of  our  present  knowledge 
with  regard  to  the  disintegration  of  the  radium  atom.  At 
each  stage  we  recognise  without  difficulty  (1)  expelled  radiant 
particles,  by  means  of  their  kinetic  energy ;  (2)  unstable  tran- 
sition-forms, on  account  of  their  radio-activity.  Independently, 
helium  has  been  recognised  as  a  product,  on  account  of  its 
excessively  sensitive  spectrum  reaction,  and  it  is  possible  this 
may  ultimately  be  identified  with  (1).  It  is  important  to  note 
that  we  should  not  be  able  to  detect,  even  if  it  were  being 
simultaneously  produced,  any  stable,  and  therefore  (presumably) 

M2 


164  RADIO-ACTIVITY. 

known,  element  not  expelled  in  the  form  of  radiant  particles, 
unless  it  were  as  easily  detected  as  the  element  helium.  The 
interesting  experiments  of  Sir  William  and  Lady  Huggins,  in 
which  the  nitrogen  spectrum  was  detected  in  the  radium  glow, 
may  ultimately  prove  to  have  a  bearing  on  this  question. 

A  point  of  great  theoretical  importance  is  whether  the  mass 
of  the  products  of  the  disintegrating  atom  equals  the  original 
mass,  and  this  cannot  be  attacked  on  the  assumption  that  the 
recognised  products  are  the  sole  products.  On  the  other  hand, 
it  can  be  directly  tested  experimentally  by  looking  for  a 
spontaneous  change  of  weight  in  a  quantity  of  radium  kept 
under  conditions  in  which  the  products  of  disintegration 
cannot  escape.  From  the  recent  views,  dealt  with  at  the 
end  of  Chapter  III.,  on  the  nature  of  inertia  and  its  possible 
dependence  on  the  internal  energy  of  the  atom,  it  is  indicated 
that  the  law  of  the  conservation  of  mass  will  not  apply  to  the 
case  of  atomic  transformation.  Considerations  such  as  these 
serve  to  show  how  useless  it  is  to  attempt  to  find  numerical 
relations  between  the  atomic  weights  of  the  elements  of  the 
Periodic  Table.  It  is  notorious  that  all  such  efforts  have  been 
fruitless,  but  it  is  only  recently  that  the  reasons  for  the  failure 
have  been  indicated. 


CHAPTER   XL 


THE  ENERGY  OF  RADIO-ACTIVE  CHANGE. 

Energy  Evolved  by  Uranium — by  Radium — by  the  Radium  Emanation. — 
Energy  Evolved  on  Disintegration  a  million-fold  greater  than  during 
Chemical  Changes. — Average  Life  of  the  Radium  Atom. — Theoretical 
Estimate. — Experimental  Determination. — Internal  Energy  of  Radium 
Ati-m. — Probability  tliat  all  Atoms  possess  great  Internal  Atomic 
Energy. 

The  first  measurements  in  this  connection  are  those  of 
Rutherford  and  McClung  (Trans.  Roy.  Soc.,  1901,  196,  p.  55), 
who  deduced  from  a  determination  of  the  energy  required  to 
produce  an  ion  in  air,  and  from  the  number  of  ions  produced 
by  uranium,  that  1  gramme  of  uranium  oxide  must  radiate 
every  year  energy  equivalent  to  0'03  calorie  as  a  minimum 
estimate.  This  amount  would  raise  a  weight  of  water  equal  to 
that  of  the  uranium  compound  1°C.  in  30  years.  If  the  radio- 
activity of  radium  in  terms  of  uranium  is  taken  to  be  106, 
the  heat  evolved  by  this  element  would  raise  its  own  weight 
of  water  1  deg.  in  15  minutes.  This  has  proved  to  be  much 
below  the  truth. 

The  energy  emitted  by  radium  compounds  has  recently 
been  experimentally  determined  by  Curie  and  Laborde  (Comptes 
Rendus,  1903,  136,  p.  673).  Two  methods  were  employed. 
In  one  a  known  quantity  of  the  radium  compound  was  placed 
in  a  Bunsen  ice  calorimeter,  and  the  heat  evolved  in  a  defi- 
nite time  measured  by  determining  the  quantity  of  ice  melted 
in  the  usual  way.  In  the  second  the  radium  was  placed  in  a 
metal  block,  and  the  temperature  of  the  block  above  that  of 
the  surrounding  air-bath  determined.  The  radium  was  then 
replaced  by  a  coil  of  wire  that  could  be  electrically  heated, 
and  the  current  measured  which  it  was  necessary  to  pass 
through  the  coil  to  maintain  it  at  the  same  temperature 
above  the  surroundings  as  was  maintained  by  the  radium. 


166  RADIO-ACTIVITY. 

The  quantity  of  heat  supplied  in  unit  time  could  be 
determined  from  the  electrical  measurements,  and  this  was 
equal  to  that  supplied  by  the  radium.  Curie  and  Laborde 
give,  as  the  result  of  their  measurements,  that  1  gramme  of 
radium  evolves  about  100  gramme-calories  of  heat  per  hour, 
or  sufficient  to  raise  its  own  weight  of  water  1  deg.  in  36 
seconds.  In  about  40  hours  sufficient  energy  is  evolved  to 
decompose  its  own  weight  of  water  completely  into  its  con- 
stituents, hydrogen  and  oxygen.  Now,  in  this  reaction  the 
change  of  hydrogen  and  oxygen  into  water,  or  vice  versa, 
more  energy  is  involved,  weight  for  weight  of  matter 
employed,  than  in  any  other  chemical  reaction  known.  HenceT 
in  two  days  the  radio-active  change  of  radium  evolves  more 
energy  than  is  ever  evolved  by  the  same  quantity  of  matter  in 
ordinary  chemical  change.  The  change  of  radium,  however, 
in  two  days  is  quite  inappreciable  by  ordinary  measurement. 
On  the  disintegration  theory,  it  is  to  be  expected  that  the 
heat-emission  from  a  radio-active  substance  will  be  proportional 
to  the  a  radiation  from  it.  For  the  latter  is  a  measure  of  the 
amount  disintegrating.  This  conclusion  has  been  directly 
proved  by  Rutherford  and  Barnes  (Phil.  Mag.,  1904,  VI.,  7, 
p.  202).  They  measured  the  heat  effect  from  a  quantity  of 
solid  radium  bromide  which  had  been  kept  for  some  weeks  in 
the  solid  state.  They  then  removed  the  emanation  by  heating 
it,  and  condensed  the  emanation  by  liquid  air  in  a  glass  tube, 
which  was  then  sealed.  They  found  that  the  heat  evolution 
from  the  radium  was  much  reduced  by  this  treatment,  arid 
decreased  further  for  the  first  few  hours,  as  the  imparted 
activity  in  the  radium  compound  decayed,  until  a  minimum  of 
about  25  per  cent,  of  the  original  was  attained.  From  this 
point  the  evolution  of  heat  commenced  to  slowly  increase.  The 
emanation  sealed  up  in  the  glass  tube  gave,  on  the  other  handy 
a  large  heat  evolution,  so  that  the  sum  total  of  the  heat  given 
out  by  the  radium  and  the  emanation  together  was  at  any 
time  equal  to  that  given  out  by  the  radium  originally.  In 
a  few  hours  after  separation  the  heat  effect  of  the  emanation, 
and  of  the  activity  imparted  by  it  to  the  walls  of  the  tube, 
equalled  75  per  cent,  of  that  originally  given  by  the  radium. 
The  heat  effect  from  this  point  slowly  decreased  at  the  same 
rate  as  the  activity  of  the  emanation  decayed.  Therefore,  the 


THE  ENERGY  OF  RADIO-ACTIVE  CHANGE.          167 

heat  effect  is  at  all  times  proportional  to  the  a-radiation,  and 
follows  the  same  changes  as  the  latter  when  the  emanation  is 
removed.  The  actual  quantity  of  emanation  obtainable  from 
radium  is  almost  infinitesimally  small,  being  for  any  available 
quantity  of  radium  compound  a  practically  invisible  bubble  of 
gas.  It  was  deduced  on  p.  162  that  the  volume  of  emana- 
tion present  in  1  gramme  of  radium,  when  equilibrium  is 
attained,  is  about  1*3  cubic  mm.  It,  together  with  the  imparted 
activity  produced  by  it,  evolves  75  calories  per  hour,  and 
the  total  evolution  of  heat  is  given  by  multiplying  the 
emission  per  hour  by  the  average  life  I/A,  expressed  in  hours, 
and  is,  therefore,  about  10 4  calories.  1  cubic  cm.  would,  there- 
fore, evolve  7x10°  calories  during  its  complete  life.  This  is  a 
purely  experimental  result,  and  is  independent  of  all  hypo- 
thesis. The  energy  liberated  on  explosion  by  1  cubic  cm.  of 
hydrogen  and  oxygen,  in  the  proportion  required  to  form 
water,  is  about  2  calories.  The  energy  of  the  disintegration 
of  the  radium  emanation  is  thus  several  million  times  greater 
than  the  energy  of  explosion  of  an  equal  volume  of  hydrogen 
and  oxygen.  This,  it  would  seem,  is  an  unanswerable  argu- 
ment in  favour  of  the  view  that  the  energy  emitted  by  radium 
comes  from  the  internal  energy  of  the  atom  when  it  disinte- 
grates, and  is  not  derived  from  a  hypothetical  external  source, 
which  radium  alone  has  power  to  respond  to. 

The  argument  may  be  pushed  one  stage  further.  It  is 
probable  that  by  far  the  greater  part  of  the  energy  liberated 
in  disintegration  appears  as  the  kinetic  energy  of  the 
u-particle  expelled.  For,  if  one  atom  of  radium  disintegrates 
into  one  a-particle  and  one  atom  of  emanation,  by  a  well- 
known  mechanical  principle,  after  disintegration  the  momentum 
of  the  a-ray  particle  will  be  equal  and  opposite  to  that  of  the 
emanation  particle.  The  velocities  will,  therefore,  be  inversely 
proportional  to  the  masses.  From  evidence  of  the  rate  of 
diffusion  of  the  emanation,  which  has  been  discussed  (p.  157),  it  is 
probable  that  the  atom  of  emanation  is  at  least  50  times  heavier 
than  that  of  the  a-particle,  and  the  velocity  of  the  latter  will, 
therefore,  be  50  times  greater.  The  kinetic  energy  is,  however, 
proportional  to  the  square  of  the  velocity,  so  that  the  kinetic 
energy  of  the  a-particle  must  be  at  least  50  times  greater  than 
that  of  the  rest  of  the  atom.  If  more  than  one  a-particle  is 


168  RADIO-ACTIVITY. 

produced  by  the  disintegration,  the  probable  result  would  be 
that  an  even  less  proportion  of  the  energy  is  received  by  the 
heavier  particle,  for  the  expulsion  of  two  a  particles  in  opposite 
directions  would  neutralise  each  other's  effect,  leaving  the  heavy 
nucleus  at  rest.  Hence,  on  the  disintegration  theory,  all  but 
a  few  per  cent,  of  the  heat  effect  should  be  due  to  the  bombard- 
ment of  the  matter  of  the  radium  and  the  walls  of  the  contain- 
ing vessel  by  the  a  particles  expelled. 

The  energy  emitted  by  one  gramme  of  radium  per  hour  is 
100  calories  =  4-  2  x  109  ergs.  This  equals  the  kinetic  energy, 
|wt;2,  of  the  a-particles  expelled,  where  m  is  the  mass  of  the 
a  particles  expelled  per  hour  and  v  their  velocity.  Since,  from 
the  measurements  of  Rutherford  (p.  79),  v  =  2'5x  10°  cm.  per 
second,  we  can  find  m.— 4-2  x  109  =  im(2-5  x  109)2.  The  mass 
of  the  a  particles  expelled  from  a  gramme  of  radium  per  hour  is, 
therefore,  1*34  x  10~9  gramme,  and  per  second  is  3*73  x  10~13. 

Average  Life  of  the  Radium  Atom. — To  find  the  mass  of 
radium  breaking  up  per  second  it  is  necessary  to  know  how 
many  a  particles  are  expelled  from  each  atom.  Now,  in  the 
case  of  radium  there  is  known  to  be  at  least  four  stages  in 
the  disintegration,  each  of  which  occurs  with  the  expulsion 
of  at  least  one  a  particle.  Hence  four  is  the  minimum  possible 
number  of  a  particles  produced  from  each  atom  in  this  case. 
The  mass  of  the  a  particle  is  1*6  (H  =  l),  and  at  least  four, 
representing  a  total  mass  of  6 '4,  are  expelled  from  each 
radium  atom  of  mass  225.  Hence  the  mass  of  the  radium 
breaking  up  per  gramme  per  second  is  given  by  (225  -=-6 -4) 
x  3-73  x  10~13=  1-3  x  10~n  gramme.  This  is  a  maximum 
estimate  of  the  proportion  of  the  radium  breaking  up  per 
second,  which  by  definition  is  A,  the  radio-active  constant  of 
radium.  The  average  life  is  I/A,  and  is,  therefore,  about 
2,450  years,  as  a  minimum  estimate. 

A  completely  independent  determination,  which  is  free  from 
any  important  assumption,  follows  from  the  volume  of  the 
emanation.  The  assumptions  made  are :  (1)  That  the 
molecule  of  the  emanation  is  monatomic,  like  argon,  which 
seems  likely,  since  it  shows  no  power  of  chemical  combination ; 
(2)  That  only  one  atom  of  emanation  results  from  one  atom 
of  radium,  which  follows  from  the  high  density,  as  shown  by 
the  co-efficient  of  diffusion,  and  from  (1).  If  the  density  is 


THE  ENERGY  OF  RADIO-ACTIVE  CHANGE.         169 

taken  as  80  (H  =  l),  the  atomic  weight  must  be  160,  and  not 
more  than  one  atom  of  emanation  could  be  produced  from  one 
atom  of  radium  (225).  These  assumptions,  even  if  untrue, 
can  hardly  affect  the  order  of  the  result  obtained. 

One  gramme  of  hydrogen  occupies  the  volume  of  11-2 
litres,  and,  if  its  molecule  were  monatomic,  would  occupy 
22 '4  litres.  One  gramme  of  radium,  if  it  could  be  obtained 
in  the  form  of  a  monatomic  gas,  would,  therefore,  occupy  a 
volume of22*4-j-225  =  0'l  litre  =  105  cubic  mm.  One  gramme  of 
radium  produces  1  -3  cubic  mm.  of  emanation  in  5*3  days,  and,  by 
Avogadro's  law,  therefore,  the  ratio  of  the  number  of  atoms  of 
emanation  to  the  number  of  atoms  of  radium  is  1'3  :  100,000, 
which  gives  the  proportion  of  the  radium  undergoing 
disintegration  in  5'3  days.  The  value  of  A  is,  therefore, 
about  2'SxlO-11,  and  the  average  life  of  the  atom  is  1,150 
years.  Although  this  value  is  not  likely  to  be  very  exact, 
and  must  be  regarded  as  a  preliminary  estimate,  it  is  pro- 
bable that,  at  least,  it  approximates  to  the  truth.  This  is 
in  fair  agreement  with  the  theoretical  estimate  obtained 
indirectly  above,  and  the  result  indicates  that  probably  the 
assumption  that  only  one  a-particle  is  expelled  at  each  disinte- 
gration is  correct.  For,  if  more  than  one  were  expelled,  the 
theoretical  estimate  would  be  still  farther  removed  from  the 
experimental  value.  The  same  would  hold  if  the  emanation 
were  considered  to  possess  a  polyatomic  molecule.  On  the 
other  hand,  if  the  single  atom  of  radium  were  assumed  to 
produce  two  atoms  of  the  emanation,  the  value  found  for  the 
average  life  of  the  radium  atom  must  be  doubled,  and  this 
would  agree  almost  exactly  with  the  theoretical  estimate.  But 
the  data  are  not  sufficiently  accurate  to  render  this  deduction 
legitimate,  for,  from  the  nature  of  the  experiment,  the  volume 
found  for  the  emanation  was  the  maximum  volume,  and  any 
trace  of  other  gas  present  as  impurity  would  tend  to  make  the 
value  greater  than  it  should  be.  If  the  nature  of  the  data  are 
considered,  the  agreement  between  the  two  values  must  be 
considered  to  be  as  close  as  could  reasonably  be  expected. 

Internal  Energy  of  the  Radium  Atom. — Since  2'8  x  10~n  is  the 
fraction  of  the  radium  disintegrating  per  second,  and  1  gramme 
of  radium  evolves  100  calories  per  hour,  it  follows  that 
100  calories  result  from  the  disintegration  of  3,600  x  2-8  x  10~n 


170  RADIO-ACTIVITY. 

grammes  of  radium.  The  total  energy  evolved  in  the  complete 
disintegration  of  1  gramme  of  radium  is,  therefore,  about  1(F 
calories.  The  energy  produced  in  the  formation  of  1  gramme 
of  water  from  its  elements  is  approximately  4  x  103.  We  thus 
see  that,  in  the  atomic  disintegration  of  radium,  about  250,000 
times  as  much  energy  is  produced  as  is  produced  in  any  known 
chemical  change.  The  question  naturally  arises  whether  this 
energy  is  only  possessed  by  radium,  or  is  not  common  to  heavy 
elements  in  general.  The  normal  character  of  radium  as  a 
chemical  element,  and  its  close  resemblance  to  the  other  elements 
of  the  same  family  which  are  not  radio-active — viz.,  barium, 
strontium,  &c. — points  to  the  conclusion  that  the  internal 
atomic  energy  of  the  inactive  heavy  elements  is  probably  of 
the  same  order  of  magnitude  as  in  radium,  but  the  absence  of 
change  in  these  cases  prevents  our  obtaining  any  information 
concerning  it.  In  the  case  of  uranium  and  thorium,  the 
internal  energy  must  be  regarded  as  of  a  similar  order  of 
magnitude  to  that  of  radium,  and  their  relatively  feeble  radio- 
activity is  to  be  ascribed  solely  to  their  slow  rate  of  disintegra- 
tion. These  considerations  force  us  to  the  conclusion  that 
there  is  associated  with  the  internal  structure  of  the  atom  an 
enormous  store  of  energy  which,  in  the  majority  of  cases, 
remains  latent  and  unknowable.  For  the  heaviest  elements 
the  property  of  spontaneous  atomic  disintegration  reveals  its- 
existence  and  enables  its  amount  to  be  calculated. 


CHAPTER   XII. 


ANTICIPATIONS. 

Five  Main  Lines  of  Enquiry  at  Present  Indicated — (1)  Tlie  Maintenance  oj 
Radium  and  Polonium  in  the  Radio-active  Minerals. — The  View  that 
these  Elements  are  Transition-forms. — Evidence  with  regard  to 
Polonium. — The  Present  Stage  of  the  Enquiry  with  regard  to  Radium. 
—(2)  The  Nature  of  the  Atom.— The  Meaning  of  the  Law  of  Radio- 
active Change. — The  requirements  of  Chemistry. — Differences  between 
Individual  Atoms  of  the  same  Element. — Deduction  that  the  Component 
Parts  of  the  Atoms  must  be  in  Violent,  Irregular  Motion.  —  (3)  The  Law 
of  the  Equivalence  of  Electric  Charges.— Its  Applicability  to  Sub- 
*  atomic  Change. — Evidence  of  the  Simultaneous  Production  of  two 
Positive  Charges.— The  Problem  of  Chemical  Valency. — (4)  The  Age 
of  the  Earth. — The  Controversy  between  Physics  and  Biology. — 
Lord  Kelvin's  Estimate. — Consideration  of  the  three  Arguments  in  light 
of  Recent  Knowledge. — A  Great  Extension  of  the  Older  Estimates 
allowable. — Estimate  of  Maximum  Age  of  the  Earth,  based  on 
Radio-active  Considerations. — Wio-year  Limit. — (5)  The  possibility  of 
the  Reconstruction  of  the  Heavier  Elements. — The  Difficulty  of  the 
Source  of  Available  Energy. — The  Applicability  of  the  Second  Law  of 
Thermodynamics  to  Sub-atomic  Changes. — Clerk -Maxwell's  "  Sorting 
Demon." — The  Two  Alternative  Views.  — The  possibility  of  Cyclic 
Evolution  in  Cosmical  Processes. 

The  theory  of  atomic  disintegration,  at  first  put  forward  on 
radio-active  evidence  alone,  has  received  such  a  full  measure  of 
direct  experimental  confirmation  that  it  will  be  well  to  devote 
this  final  chapter  to  the  consideration  of  some  features  of 
the  theory  which  are  clearly  indicated  at  the  present  time,  but 
which  have  not  yet  received  experimental  verification.  The 
most  important  of  these  have  to  do  (1)  with  the  means  by 
which  the  quantity  of  radium  (and  polonium)  is  maintained  in 
the  earth,  (2)  with  the  nature  of  the  chemical  atom  and 
closely  bound  up  with  it,  (3)  the  question  as  to  whether 


172  RADIO-ACTIVITY. 

the  equivalence  of  electric  charges  is  maintained  during  atomic 
disintegration,  (4)  the  error  in  the  older  physical  estimates  of 
the  possible  age  of  the  earth,  and  (5)  the  possibility  of  a 
continuous  cycle  of  cosmical  evolution  to  which  the  second  law 
•of  thermo-dynamics  does  not  apply. 

The  determination  of  the  average  life  of  the  radium  atom 
(p.  169)  shows  clearly  that  in  the  course  of  only  a  few 
thousand  years  the  quantity  of  radium  existing  in  a  mineral 
would  be  reduced  to  practically  nothing  unless  a  concomitant 
process  of  reproduction  were  taking  place  within  the  mineral. 
In  the  case  of  polonium  the  same  would  occur  in  the  course  of 
only  a  few  years.  The  readiest  explanation  is  to  suppose  that 
the  radium  and  polonium  atoms  are  themselves  really  meta- 
bolons  (in  the  sense  of  having  been  themselves  produced 
Toy  sub-atomic  change),  but  with  a  somewhat  more  extended 
period  of  life  than  in  the  other  cases.  From  this  point  of 
view  radium  and  polonium  are  merely  slow-changing  transition- 
forms  in  the  disintegration  of  a  heavier  and  much  more 
slowly  changing  parent  radio-element,  as,  for  example,  uranium 
or  thorium. 

Consider  the  case  of  a  disintegration  series  in  which  there  is 
.a  parent  element,  A,  disintegrating  at  an  excessively  slow  rate 
through  successive  more  rapidly  changing  transition-forms, 
B,  C,  D,  E,  the  respective  rates  of  change  being  AA,  AB,  &c, 
If  NA,  NB,  &c.,  represent  the  number  of  atoms  of  each  type 
present  when  the  condition  of  radio-active  equilibrium  is 
attained,  the  total  radio-activity  is  the  sum  of  AANA,  ABNB, 
&c.  A  little  consideration  will  show  that  AANA  =  ABNB 
=  ACNC,  &c.,  since  at  equilibrium  the  amount  of  each  type 
changing  equals  the  amount  produced.  In  this  reasoning  it  is 
.assumed  that  one  atom  of  A  produces  one  atom  of  B,  &c. 
Hence  the  amount  of  each  transition-form  accumulating  is 
inversely  proportional  to  its  rate  of  change  or  directly  pro- 
portional to  its  average  life.  If  the  parent  element  A  exists 
in  a  natural  mineral  the  quantities  of  B,  C,  D,  &c.,  present  in 
the  mineral  will  be  proportional  to  their  average  lives. 

Suppose  now  that  B,  C,  D  are  rapidily  changing  types,  and 
that  E  is  a  very  slowly  changing  type.  It  follows  that  when 
the  mineral  is  separated  into  its  various  constituents  by 
•chemical  analysis,  E  will  not  appear  as  a  transition-form  like 


ANTICIPATIONS. 

B,  C,  D,  but  as  a  new  radio-element  whose  activity  is  sensibly 
permanent.  Whether  E  is  detected  by  its  ordinary  chemical 
reactions  depends  simply  on  its  quantity,  and  this  we  have 
seen  is  the  greater  the  longer  its  average  life.  It  thus  appears 
that  the  existence  of  a  slow-changing  transition-form  at  the 
end  of  a  series  would  probably  be  overlooked  in  work  with 
only  small  quantities  of  material,  for  both  its  radio-activity 
and  actual  quantity  will  be  relative!}'  small.  But  when  tons 
of  a  mineral  containing  the  parent  element  are  worked  up 
these  forms  may  be  separated,  and  will  appear  as  new  power- 
fully radio- elements  possessing  sensibly  permanent  activity. 

The  application  of  these  considerations  leads  to  the  view 
that  polonium  and  radium  may  both  be  transition-forms  in  the 
disintegration  series  of  uranium.  The  rate  of  change  of 
uranium,  judging  by  the  radio-activity  of  unit  weight  compared 
to  that  of  radium,  is  about  one  million  times  less  than  the  rate 
of  change  of  radium.  Hence  in  a  mineral  in  radio-active- 
equilibrium  the  quantity  of  uranium  should  be  about  one 
million  times  the  quantity  of  radium.  The  rate  of  change  of 
polonium  is  about  one  thousand  times  that  of  radium,  and  the 
quantity  of  polonium  should  be  about  one-thousandth  of  the 
quantity  of  radium,  or  10~9  of  the  quantity  of  uranium.  The 
work  of  Mme.  Curie,  Giesel  and  Marckwald  on  the  proportion 
of  radium  and  polonium  in  pitchblende  (Chapter  II.)  sup- 
ports the  view  that  radium  and  polonium  are  produced  from 
uranium. 

Consider,  first,  the  case  of  polonium,  and  what  would  occur 
if  it  were  being  produced  as  a  transition-form  in  a  disintegra- 
tion series.  Owing  to  its  comparatively  slow  rate  of  change, 
the  activity  of  the  polonium  for  short  periods  of  production 
would  be  very  feeble  compared  with  the  activity  of  the  earlier- 
produced  and  more  rapidly  changing  transition-forms  in  the 
same  series.  But  if  the  process  of  production  were  extended 
for  some  years,  the  activity  of  the  polonium  would  increase 
to  an  equilibrium  value  comparable  with  that  of  the  other 
types.  This  suggests  that  possibly  Mme.  Curie's  discovery 
of  the  "induced  activity  of  radium  with  slow  rate  of  dissipation" 
(p.  140)  is  caused  by  polonium,  and  that  this  type  results  as- 
the  product  of  the  change  of  the  matter  causing  the  ordinary 
imparted  activity  of  radium.  Consider  some  definite  quantity 


174  RADIO-ACTIVITY. 

of  the  latter  changing  into  polonium.  After  the  change  is 
complete  the  relative  activities  of  the  polonium  and  the 
matter  producing  it  may  be  expected  to  be  of  the  same  order 
as  the  relative  rates  of  change  of  the  two  types.  Since  for 
polonium  A  =  2  x  1 0~8,  a'nd  for  the  last  stage  of  the  imparted 
activity  matter  A  =  4xlO~4,  the  activity  of  the  polonium 
should  be  about  5xlO~5,  or  -oiyihnjth  of  that  of  the  imParted 
activity  matter  producing  it.  Now  Mme.  Curie  found  that 
the  residual  activity  left  after  the  ordinary  imparted  activity 
of  radium  had  completely  decayed  was  about  ^tr^th.  °^  ^ne 
original  activity.  The  agreement  is  certainly  striking,  but  it 
must  be  borne  in  mind  that  sufficient  data  are  not  available 
as  to  how  the  above  experimental  value  was  obtained  to  make 
it  clear  whether  these  considerations  hold.  The  ratio  of  the 
two  activities  must  be  determined  for  only  a  short  period  of 
production  of  the  imparted  activity.  If  a  negatively- charged 
wire  were  left  in  a  vessel  containing  the  emanation  for 
several  weeks  until  the  latter  had  completely  changed,  the 
residual  activity  left  on  the  wire  will  represent  the  result  of 
the  change  of  the  total  emanation,  and  the  ratio  of  the  initial 
activity  of  the  emanation  to  that  of  the  residual  activity  will 
then  be  the  ratio  of  the  rate  of  change  of  the  emanation  (not 
the  imparted  activity)  to  the  rate  of  change  of  polonium,  and 
would  be  far  higher  than  -j^^oo-th.  If,  on  the  other  hand,  the 
wire  had  been  exposed  to  the  emanation  for  only  an  hour  or 
two,  then  Mme.  Curie's  result  is  to  be  expected. 

The  view  that  polonium  is  a  disintegration  product  of 
radium  is  at  first  sight  strongly  confirmed  by  some  experiments 
of  Giesel  (Ber.  d.  Deutsch.  Chem.  Ges.,  1903,  p.  2,368). 
Giesel  repeated  the  method  applied  by  Marckwald  for  the 
separation  of  polonium  from  pitchblende,  and  kept  pieces  of 
bismuth,  palladium  and  platinum  respectively  in  solutions  of 
powerful  radium  preparations.  He  obtained  a  sensibly 
permanent  activity,  consisting  only  of  a  rays,  from  the  metals 
after  they  had  been  freed  from  every  trace  of  radium  and  were 
left  to  themselves  for  a  few  days.  He  further  found  that  the 
radio-active  deposit  was  formed  on  a  wire  held  in  the  air 
above  the  solution,  which  is  in  accordance  with  the  above 
conclusion  that  polonium  is  produced  from  the  matter  causing 
the  induced  activity,  which  itself  is  produced  from  the 


A  NT  1C  IP  A  TIOXS.  175 

emanation,  which  would  diffuse  into  the  air  above  a  radium 
solution. 

Now  the  characteristic  of  the  polonium  radiation  is  that  it 
consists  only  of  a-rays.  In  some  experiments  performed  by 
the  Author  on  the  character  of  the  radiation  from  the  residual 
activity  left  by  the  radium  emanation,  it  was  found  that  both 
J3  and  a-rays  were  present,  although  the  former  were  not  very 
obvious  and  the  ratio  of  the  two  types  was  much  less  than  in 
the  case  of  uranium.  It  is  possible  that  these  /3-rays  result 
from  intermediate  products,  and  that  they  will  be  found  to 
have  decayed  when  the  radiations  are  re-examined  after  a 
sufficient  interval.  Or,  possibly,  we  have  here  a  case  of  the 
simultaneous  production  of  two  different  types  of  active 
matter  in  the  same  disintegration.  But  on  this  account  the 
question  as  to  whether  polonium  is  the  last  type  produced  in 
the  disintegration  of  radium  still  remains  open.  A  conclusive 
proof  could  only  be  obtained  by  comparing  the  rate  of  decay 
of  the  activity  of  the  two  types,  and  this  must  of  necessity 
take  several  years. 

With  regard  to  the  case  of  radium,  the  Author  has  recently 
found  that  the  quantity  of  radium  produced  by  uranium  in 
the  first  year  of  accumulation  is  not  TuT&roth  part  of  the 
quantity  that  would  be  expected  to  result,  assuming  a  direct 
change  of  uranium  into  radium.  The  method  was  to  com- 
pletely free  1kg.  of  uranium  nitrate  from  radium  by  precipi- 
tating barium  in  its  solution  by  means  of  sulphuric  acid. 
It  was  found  that  the  barium  sulphate  drags  down  even  the 
last  traces  of  radium  present.  The  test  for  the  latter  was  the 
emanation,  which  was  allowed  to  accumulate  about  four 
days  in  a,  closed  vessel  before  being  sent  into  an  electroscope. 
As  uranium  does  not  give  an  emanation,  the  presence  of  radium 
could  thus  be  very  simply  detected.  After  the  lapse  of  about 
a  year  the  quantity  of  radium  present,  using  the  same  test, 
was  found  to  be  less  than  10~n  gramme.  If  the  average  life 
of  uranium  atom  is  assumed  to  be  109  years,  in  1kg.  of  the 
nitrate  about  5  x  10" 7  gramme  would  change  per  year,  and 
if  this  had  turned  into  radium  it  would  have  been  100,000 
times  as  much  as  could  have  been  detected  under  the  condi- 
tions of  measurement.  Hence,  the  source  of  radium  in 
minerals  still  remains  a  completely  open  question.  It  is 


176  RADIO-ACTIVITY. 

possible  that  if  an  intermediate  form  (other  than  uranium  X, 
which  changes  too  quickly  to  affect  the  result)  exists  between 
uranium  and  radium — as,  for  example,  actinium — with  a  very 
slow  rate  of  change,  that  it  would  account  for  the  above 
result.  The  same  explanation  might  also  account  for  the 
known  fact  that  some  minerals  containing  uranium  appear  to 
contain  but  little  radium,  for  if  the  matter  passed  through  a 
gaseous  stage  (or  an  emanation)  before  the  radium  was  pro- 
duced, the  amount  of  the  latter  present  in  a  mineral  would 
depend  upon  whether  the  emanation  was  able  to  escape  or  was 
accumulated  by  the  mineral.  But  it  seems  more  probable  from 
the  available  evidence  that  uranium  is  not  the  parent  element 
of  radium,  and  the  question  remains  at  present  unsettled. 

The  Nature  of  the  Atom. — It  has  been  pointed  out  that  the 
law  followed  by  radio-active  change,  that  a  constant  proportion 
of  the  total  number  of  atoms  changes  in  unit  time,  is  that 
followed  by  a  monomolecular  chemical  reaction.  It  is  difficult 
to  find  a  true  case  of  monomolecular  reaction  analogous  to 
radio-active  change.  Many  so-called  monomolecular  reactions 
are  polymolecular,  in  which  one  molecule  is  present  in  over- 
whelming preponderance,  as  in  the  historic  case  for  which  the 
law  was  discovered  by  Wilhelmy — the  hydrolysis  of  cane 
sugar  in  aqueous  solution. 

The  true  monomolecular  reactions  which  have  been  studied 
accurately  are  nearly  all  endothermic,  i.e.,  proceed  with  absorp- 
tion of  heat,  instead  of  with  evolution  as  in  radio-active  change. 
An  example  is  the  dissociation  of  the  diatomic  iodine  mole- 
cule into  single  atoms.  The  course  of  a  monomolecular 
reaction  proceeding  with  heat  evolution  cannot  be  accurately 
studied,  because  the  heat  produced  itself  increases  the  rate  of 
change,  which  often  proceeds  to  explosion,  as  in  the  case  of 
the  decomposition  of  any  high  explosive.  Chemistry,  there- 
fore, does  not  offer  much  assistance  in  the  inquiry  as  to  the 
physical  meaning  of  the  law  of  radio-active  change.  Since 
external  conditions  do  not  affect  the  rate  of  change,  and  the 
change  proceeds  with  evolution  of  energy,  the  question 
naturally  arises  as  to  the  controlling  influence  which  regulates 
the  speed  of  the  reaction  and  ensures  that  only  a  certain 
fraction  of  the  total  number  of  systems  shall  change  in  unit 
time.  In  short,  why  do  not  all  atoms  of  a  radio-active 


ANTICIPATIONS.  177 

.substance  break  up  together,  since  all  ultimately  break  up,  and 
in  so  doing  evolve  vast  quantities  of  energy  ?  Obviously  there 
must  exist  individual  differences  between  the  atoms,  and,  since 
temperature  and  other  influences  are  without  effect,  these 
differences  cannot  be  merely  due  to  external  differences,  such 
as  varying  speed  of  translation,  &c.  They  must  be  sought  for 
within  the  atom  structure. 

This  is  an  absolutely  fundamental  conclusion  in  its  bearing 
on  chemistry.  The  atomic  hypothesis  of  Dalton,  although 
frequently  associated  vaguely  with  other  quite  alien  considera- 
tions as  to  the  stability  of  atoms,  differed  in  one  essential 
particular  from  earlier  atomistic  notions,  prevalent  as  far  back 
as  recorded  history.  The  atoms  of  one  type  of  matter,  or 
element,  according  to  the  Daltonian  theory,  are  all  alike,  and 
different  in  essential  and  recognisable  properties  from  the 
atoms  of  any  other  type.  Until  the  discovery  of  radio- 
activity this  held  true  without  exception,  although  in  the  case 
of  the  rare-earth  elements  the  differences  are  so  slight — the 
different  types  seeming  to  merge  almost  continuously  into  one 
another — that  the  validity  of  the  law  might  be  questioned.  In 
the  case  of  the  radio-active  elements,  although  for  ordinary 
properties  they  may  be  considered  to  be  among  the  best  defined 
and  most  sharply  distinguished  of  the  Daltonian  atoms,  their 
additional  property  of  disintegration  reveals  an  undoubted 
difference  between  the  individual  atoms.  An  example  will 
serve  to  emphasise  the  point.  It  was  shown  on  p.  125  that 
the  average  life  of  each  typical  unstable  atom  is  a  constant 
well  suited  for  its  experimental  identification.  Yet,  if  a 
quantity  of  a  homogeneous  radio-active  element  is  considered 
from  any  instant,  some  of  the  atoms  break  up  in  the  first 
second,  and  their  life  is  therefore  something  less  than  a  second, 
whereas,  even  after  infinite  time,  some  atoms  will,  theoretically, 
survive.  The  actual  life  of  an  atom  of  radium  has  all  values 
between  zero  and  infinity.  This  precludes  the  idea  that  the 
atom  structure  is  similar  to  an  isolated  solar  system  in  which 
the  parts  go  through  cycles  of  motion  unperturbed  by  external 
agencies.  If  disintegration  occurred  at  a  certain  orientation  of 
the  planets,  then,  although  in  a  collection  of  a  large  number  of 
such  systems  all  possible  orientations  might  be  imagined  to  exist 
at  the  same  instant,  and  some  would  therefore  disintegrate 


178  RADIO-ACTIVITY. 

immediately,  the  maximum  life  of  any  system  could  not  exceed  a 
certain  value,  represented  in  an  elementary  case  by  one  complete 
cycle.  The  proportion  of  the  total  changing  would  increase  with 
lapse  of  time,  for  a  definite  number  rather  than  a  definite 
fraction  would  come  into  the  unstable  position  every  second. 
The  average  life  of  the  systems  would  depend  on  the  extent  of 
their  previous  history,  whereas  the  most  fundamentally 
remarkable  feature  of  radio-active  systems  is  that  the  average 
life  is  a  constant,  independent  of  the  length  of  time  the 
systems  have  survived.  Hence,  the  idea  of  such  solar  systems 
is  excluded.  It  is,  also,  quite  untenable  on  purely  chemical 
grounds.  For,  if  cycles  of  motion  existed — the  orientation  of 
the  atom  passing  through  a  defined  course  and  returning  again 
to  its  initial  position  after  a  finite  time — chemical  differences 
should  exist  between  the  individual  atoms,  and  it  should  be 
easy  by  chemical  analysis  to  separate  the  homogeneous  elements 
into  groups  of  similar  phase,  the  difference  of  the  groups  dis- 
appearing slowly  with  lapse  of  time.  Possibly  such  an  idea, 
or  a  development,  might  apply  to  the  case  of  the  rare- earth 
alements  already  alluded  to. 

The  question  remains,  Is  it  possible  to  frame  a  first  view 
of  atomic  structure  which  shall  be  consistent  with  the  law  of 
radio-active  change  and  the  well-known  demands  of  chemistry  ? 
It  is  simpler  to  consider  the  latter  first.  We  have  seen  that 
no  slow-recurring  cycle  of  internal  motion  can  be  entertained. 
Nevertheless,  individual  differences  must  be  admitted.  There 
would  seem  no  escape  from  the  conclusion  that  the  internal 
parts  of  an  atom  must  be  in  extremely  rapid  motion,  so  that, 
even  although  chemical  differences  exist  between  atoms  of 
different  phase,  and  these  are  capable  of  separation  by  suitable 
processes,  the  reversion  of  the  opposed  phases  to  the  general 
average  must  take  place  so  quickly  that  no  difference  of  pro- 
perties between  the  two  parts  after  separation  is  detectable 
Hence  the  conclusion  that  the  internal  structure  of  an  atom  is 
in  violent  motion,  which  is,  of  course,  one  of  the  first  principles 
of  the  electronic  view  of  the  constitution  of  matter,  can  be 
deduced  from  quite  independent  considerations. 

The  bearing  of  the  idea  on  chemical  theory  cannot  be  discussed 
here.  It  will  be  seen  at  once  that  chemical  properties  must  be 
regarded  as  average  properties,  the  individual  atoms  varying 


ANTICIPATIONS.  179 

continuously  in  nature  between  certain  limits,  which  need  not 
necessarily  be  narrow.  Whether  this  will  assist  the  elucidation 
of  any  of  the  problems  of  chemistry  remains  for  the  future. 

The  position  is  reached  that  rapidly  recurring  motions  within 
the  atom,  giving  rise  to   orientations   exhibiting   individual 
differences,  but  reverting  to  a  general  average  in  extremely 
short  intervals  of  time  after  separation,  can,  at  least,  be  enter- 
tained.    The  problem  remains  as  to  how  the  slow  periods  of 
radio-active  changes  are  brought  about.     The  mutual  action 
of  the  systems  on  one  another  would  seem  to  be  excluded, 
otherwise  radio-active  change  should  vary  with  concentration. 
Hence  the  case  may  be  considered  as  between  the  single  atom 
and  its  action  on  the  universal  ether.     It  would  appear,  from 
the  nature  of  the  case,  that  the  cause  of  the  disintegration  can- 
not be  due  to  any  regular  or  continuous  action  between  the 
matter  and  the  ether,  which  produces  a  permanent  effect  on 
the  internal  nature  of  the  atom.    If,  for  example,  one  supposes, 
with  Sir  Oliver  Lodge,  that  the  reaction  between  the  revolving 
parts  of  the  atom  and  the  ether  gives  rise  to  an  electromag- 
netic   wave  radiation,    which    diminishes   the    total    internal 
energy  to  a  point  below  which  disintegration  is  possible,  it  i& 
difficult  to  see  how  the  average  life  of  the  systems  surviving 
at  each  instant  is  constant  and  independent  of  the  extent  of 
their  previous  existence.     It  seems  certain  that  no  permanent 
alteration  of  a  gradual  character  can  occur  in  the  structure  of 
an  atom  with  lapse  of  time.     An  atom  that  has  just  failed  at 
one  instant  to  disintegrate  may  in  the  next  instant  be  as  far 
removed  as  any  from  the  unstable  condition.      In  other  words, 
the  action  appears  to  be  due  to  "chance  "—i.e.,  the  orientation 
assumed  at  one  instant  has  no  determining  influence  on  the 
orientation  about  to  be  assumed  at  the  next  instant.     The 
conclusion  is  thus  arrived  at  that  the  internal  movements  of 
the  atom  must  be  highly  irregular  and  cannot  follow  a  definite 
sequence  if  the  law  of  radio-active  change  is  to  hold  good. 
The  unstable  position  appears   to  be  rather  the  result  of  a 
chance  collocation  of  the  parts  than  to  be  due  to  the  operation 
of  any  simple  law.     An  analogy  might  be  drawn  from  the 
kinetic  theory  of  gases,  in  which  certain  of  the  molecules  are 
regarded  as  possessing  momentarily  much  higher  and  others 
much  lower  temperature  than  the    average,  and  the  acting 

N2 


180  RADIO-ACTIVITY. 

causes  are  so  complex  that,  although  the  proportion  of  the 
whole  at  any  temperature  may  possibly  be  calculated  when 
the  total  number  of  systems  is  exceedingly  great,  the  indi- 
vidual history  of  any  one  molecule  is  quite  indefinite.  In  a 
radio-active  substance  a  definite  fraction  of  the  total  assumes  a 
peculiar  orientation  and  disintegrates  in  each  second,  but  the 
life  of  any  single  atom  is  quite  indefinite.  The  causes  at  work 
appear  to  be  so  complex  that  the  results  can  only  at  present 
be  described  as  "  chance  "  or  "  accidental  "  happenings,  in  the 
sense  of  being  impossible  to  predict.  The  conclusion  is  reached 
that  the  structure  of  the  atom  must  be  in  excessively  rapid 
and  irregular  motion.  This  is  a  step  further  than  purely 
electrical  considerations  at  present  indicate. 

Atomic  disintegration  is  of  such  a  completely  novel  character, 
and  is  so  far  removed  from  all  ordinary  atomic  and  molecular 
phenomena,  that  none  of  the  fundamental  laws  which  regulate 
the  latter  necessarily  apply,  by  analogy,  to  the  former.  It 
was  pointed  out  on  p.  164  that  it  is  not  to  be  expected  that 
the  law  of  the  conservation  of  mass  will  hold  true  for  radio- 
active phenomena.  The  work  of  Kaufmann  may  be  taken  as  an 
experimental  proof  of  the  increase  of  apparent  mass  of  the 
electron  when  its  speed  approaches  that  of  light.  Since  during 
disintegration  electrons  are  expelled  at  speeds  very  near  that 
of  light,  which,  after  expulsion,  experience  resistance  and 
suffer  diminution  of  velocity,  the  total  mass  must  be  less  after 
disintegration  than  before.  On  this  view  atomic  mass  must 
be  regarded  as  a  function  of  the  internal  energy,  and  the  dissi- 
pation of  the  latter  in  radio-activity  occurs  at  the  expense,  to 
some  extent  at  least,  of  the  mass  of  the  system. 

The  Law  of  the  Equivalence  of  Electric  Charges.— Of  other 
generalities  which  have  found  universal  acceptance  in  the 
domain  of  molecular  physics,  the  fundamental  law  of  elec- 
tricity, that  in  an  originally  electrically  neutral  system 
positive  and  negative  electricity  always  result  together  in 
equal  quantities,  remains  to  be  considered  with  reference  to 
radio-activity.  It  must  be  understood  that  the  evidence  at 
present  available  is  little  more  than  sufficient  to  justify  the 
question  being  raised.  Although  apparently  pointing  in  one 
direction,  so  many  possibilities  arise,  which  have  not  yet  been 
critically  examined,  that  the  bearing  of  the  evidence  must 


ANTICIPATIONS.  181 

be  accepted  with  reserve.  The  main  phenomenon  of  radio- 
activity has  been  shown  to  consist  in  the  successive  expulsion 
of  positively  charged  particles  from  an  originally  neutral 
atom.  The  deduction,  therefore,  is  that  the  atomic  residue 
after  each  expulsion  should  be  left  negatively  charged.  In 
the  cases  which  have  been  most  completely  examined  it  is 
found  to  be  left  positively  charged,  so  that  it  would  appear 
that  there  is  the  simultaneous  production  of  two  positive 
charges  during  many  cases  of  atomic  disintegration.  The  best 
established  cases  are  those,  worked  out  by  Eutherford,  of  the 
change  of  the  emanations  of  thorium  and  radium  into  the 
matter  causing  the  imparted  activity.  It  was  pointed  out 
(p.  141)  that  the  emanation  atom,  whether  charged  or  not  at 
the  moment  of  formation,  rapidly  becomes  uncharged,  and  at 
the  moment  of  disintegration  must  be  considered  neutral.  In 
the  case  of  radium  the  positive  charge  carried  by  the  a  particle 
expelled  has  been  proved  by  Rutherford  by  the  direction  of 
the  deviation  of  the  particle  in  a  magnetic  field.  All  the  a 
particles  from  radium  are  deviated  as  though  positively  charged, 
and  since  about  40  per  cent  of  the  total  a  radiation  is,  under 
ordinary  circumstances,  derived  from  the  emanation,  the  a 
radiation  of  the  emanation  must  carry  a  positive  charge.  The 
matter  produced  from  the  atom  after  the  expulsion  of  the 
positive  ray  is  also  positively  charged,  and  moves  in  an  electric 
field  to  the  negative  electrode.  The  same  holds  true  for  the 
thorium  emanation  and  imparted  activity.  In  other  cases 
there  is  evidence  that  the  residual  system  is  left  positively 
charged  at  the  moment  of  the  expulsion  of  the  ray. 

Prof.  J.  J.   Thomson*   recently   performed   the   following 
experiment  :— 

A  quantity  of  radium  was  completely  enclosed  in  a  block  of 
lead  to  which  two  gold  leaves  were  externally  attached.  The 
system  was  suspended  by  a  quartz  fibre  in  a  highly  exhausted 
space.  It  was  found  that  if  the  lead  was  of  sufficient  thickness 
to  completely  absorb  the  /3  radiation  the  leaves  did  not  charge. 
With  smaller  thickness  of  lead  the  leaves  diverged  with  a  posi- 
tive charge  as  in  Strutt's  apparatus  (Fig.  19).  This  would  seem 
to  prove  that  the  positive  and  negative  electricity  produced 
during  the  disintegration  are  equal  and  neutralise  one  another. 
*  Private  communication. 


182  RADIO-ACTIVITY. 

It  must  be  pointed  out  that  no  direct  evidence  has  yet  been 
obtained  of  the  positive  charge  carried  by  the  a  particle.  Wien, 
in  some  recent  experiments,  sought  to  perform  a  similar  experi- 
ment to  that  of  M.  and  Mme.  Curie  for  the  /3-rays  of  radium 
(p.  73)  for  the  a-rays,  but  failed  to  obtain  a  positive  result. 

The  reason  why  these  considerations  are  here  introduced 
is  to  draw  attention  to  the  bearing  of  the  evidence  on  the 
problem  of  chemical  valency.  Assume,  for  the  sake  of 
example,  that  the  a  particle  is  an  atom  of  helium.  The  dis- 
ruption of  the  divalent  radium  atom  produces  two  non-valent 
atoms — helium  and  the  emanation.  This  is  such  a  novel 
change  that  the  question  may  reasonably  be  asked  whether  it 
.is  right  to  assume  that  the  electrical  neutrality  of  the  system 
is  preserved  under  these  conditions.  We  have  at  present  no 
certain  knowledge  of  the  meaning  either  of  electrical  neutrality 
or  valency.  Is  it  necessarily  to  be  assumed  that  the  older 
ideas  of  the  nature  of  the  electric  charge  will  apply  without 
modification  to  the  profound  changes  within  the  atom  that 
occur  in  radio  active  processes  1 

The  Age  of  the  Earth. — The  discovery  of  the  enormous  store 
of  available  energy  latent  in  certain  atomic  structures,  and  the 
probable  existence  of  this  energy — although  not  yet  shown  to 
be  available — in  all  atoms,  which  was  discussed  in  the  last 
chapter,  have  an  intimate  connection  with  the  problems  of 
cosmical  evolution,  and  the  requirements  of  biology  and 
geology.  Throughout  the  latter  part  of  the  last  century  a 
controversy,  as  to  the  possible  age  of  the  earth  as  a  planet 
fitted  for  habitation,  existed  between  two  schools,  represented 
by  the  physicists  on  the  one  side  and  the  biologists  on  the 
other.  Some  of  the  arguments  advanced  by  the  former  make 
strange  reading  at  the  present  time.  The  case  for  the  physi- 
cists was  presented  by  the  late  Prof.  Tait  in  some  "  Lectures 
on  Kecent  Advances  in  Physical  Science,"*  and  the  following 
extracts  may  be  quoted  :  — 

"  Thus  we  can  say  at  once  to  the  geologists,  that,  granting 
this  premiss — that  physical  laws  have  remained  as  they  are 
now  and  that  we  knoiv  of  all  the  physical  laws  that  have  been 
operating  during  that  time — we  cannot  give  more  scope  for 
their  speculations  than  about  10  or  (say  at  most)  15  million 
*  Macmillan  &  Co.,  1895. 


ANT1C1PA  TIONS.  183 

years.  But  I  dare  say  many  of  you  are  acquainted  with 
the  speculation  of  Lyell  and  others,  especially  of  Darwin, 
who  tell  us  that  for  even  a  comparatively  brief  portion  of 
recent  geological  history  three  hundred  millions  of  years  will 
not  suffice  !  We  say — so  much  the  worse  for  geology  as  at 
present  understood  by  its  chief  authorities,  for,  as  you  will 
presently  see,  physical  considerations  from  various  independent 
points  of  view  render  it  utterly  impossible  that  more  than  10 
or  15  million  years  can  be  granted."  The  sentence  here 
italicised  needs  no  further  comment,  in  light  of  present-day 
knowledge,  to  decide  the  question  in  favour  of  the  geolo- 
gists. The  demands  of  the  latter,  at  first  considered  extrava- 
gant, are  well  within  the  range  of  even  the  present  limited 
knowledge  of  the  energy  associated  with  matter. 

Lord  Kelvin  arrived  at  the  10  million  year  estimate  of  the 
past  age  of  the  earth  as  a  habitable  world  from  three  indepen- 
dent considerations.  The  first  was  based  on  the  internal  heat 
of  the  earth  and  the  rise  in  temperature  beneath  the  surface. 
From  the  temperature  gradient — about  1°C.  for  every  100ft. 
of  descent — the  loss  of  heat  from  the  centre  outward  and  the 
temperature  of  the  surface  at  any  time  previous  could  be 
calculated.  This  leads  to  the  conclusion  that  not  more  than 
about  10  million  years  ago  the  surface  of  the  earth  must  have 
been  still  molten.  This  argument  rests  on  the  assumption 
that  the  earth  was  a  self-cooling  planet,  whereas  we  now  know 
that  it  is  self-Ideating  also.  If  only  a  small  fraction  of  the 
matter  of  the  earth  possessed  the  property  of  heat-evolution 
to  only  the  same  extent  as  the  element  uranium  (which  prob- 
ably evolves  about  1  calorie  per  gramme  per  year  and  lasts 
for  thousands  of  millions  of  years),  it  will  readily  be  seen  that 
the  rise  of  temperature  from  the  surface  of  the  earth  inwards 
might  have  been  attained  by  supposing  it  to  have  originally 
been  a  cold  body  spontaneously  self-heating,  rather  than  a  hot 
body  spontaneously  self-cooling. 

The  second  argument  rested  'on  what  is  known  as  tidal 
retardation.  The  earth  rotates  faster  than  the  moon  revolves. 
The  moveable  parts  of  the  earth's  surface — the  oceans — are  at- 
tracted towards  the  moon  and  tend  to  rotate  slower  than  the  rest 
of  the  earth.  The  consequences  are  twofold.  In  attracting  the 
water  the  moon  is  itself  attracted  to  an  equal  and  opposite  extent, 


184  RADIO-ACTIVITY. 

and  its  period  of  revolution — the  lunar  month — is  decreasing.. 
The  period  of  the  earth's  rotation — the  terrestrial  day — is  being 
correspondingly  increased  by  the  friction  with  which  it  opposes 
the  tidal  movements  of  the  oceans.  At  some  distant  date  the 
earth  will  rotate  in  the  same  period  as  the  moon  revolves,  and 
the  lunar  month  and  terrestrial  day  will  be  identical.  The 
rate  of  increase  of  the  earth's  period  of  rotation  being  known, 
its  value  for  any  past  time  can  be  calculated.  The  problem 
was  to  find  the  time  at  which  the  earth  was  rotating  at  such  a 
rate  that  it  took  the  particular  shape  it  possesses  at  present. 
It  was  assumed  that  the  polar  flattening  now  existing  was  that 
prevalent  at  the  time  the  crust  of  the  earth  solidified,  and  that 
after  this  event  this  shape  has  been  retained  unaltered.  The 
result  went  to  show  that  the  diurnal  period  must  have  been 
almost  what  it  is  now  to  have  produced  the  existing  flattening, 
and  hence  it  was  argued  that  the  earth  must  have  been  fluid 
less  than  10  million  years  ago.  The  recent  work  of  Prof.  F.  G. 
Adams,  the  geologist,  on  the  flow  of  crystalline  rocks  under 
pressure,  would  seem  to  indicate  that  the  earth  could  not  be 
regarded  as  a  rigid  body  even  if  it  were  solid  to  the  centre.  It 
would  seem  more  reasonable  to  suppose  that  the  earth's  shape 
would  conform  to  its  existing  period  of  rotation,  like  a  viscous 
fluid,  exhibiting,  possibly,  a  certain  amount  of  lag. 

Even  more  important  is  the  possibility  that  the  energy  of 
planetary  rotation  is  being  maintained,  possibly  by  the  radiant 
pressure  of  light.*  This  most  recently  discovered  factor  in  the 
processes  of  cosmical  evolution  has  not  yet  been  reckoned  with, 
Like  sub-atomic  change,  it  is  one  of  those  infinitely  small 
agencies  which,  acting  over  infinite  time,  may  profoundly 
modify  the  conditions  of  the  problem  to  be  solved. 

The  third  argument  was  based  upon  the  length  of  past  time 
that  the  sun  could  have  maintained  its  radiation.  Prof. 
Tait,  in  the  lectures  referred  to,  said,  "  Take  (in  mass  equal  to 
the  sun's  mass)  the  most  energetic  chemicals  known  to  us  and 
the  proper  proportion  for  giving  the  greatest  amount  of  heat 

*  For  some  mechanical  experiments  designed  on  this  idea,  see  E.  W.  0. 
Kestel,  "Kadiant  Energy,  a  Working  Power  in  the  Mechanism  of  the 
Universe"  (Port  Adelaide,  1898).  No  mathematical  analysis  of  the  effects 
of  light  pressure  on  planetary  motion  seems  as  yet  to  have  been  under- 
taken. The  problem  is  of  extreme  interest. 


ANTICIPA  TIOXS.  185 

by  actual  chemical  combination,  and,  so  far  as  we  yet  know 
their  properties,  we  cannot  see  the  means  of  supplying  the 
sun's  present  waste  for  even  5,000  years.  .  .  .  It  is  quite 
obvious  that  the  heat  of  the  sun  cannot  possibly  be  supplied 
by  any  chemical  process  of  which  we  have  the  slightest  con- 
ception. .  .  .  This  question  is  totally  unanswerable,  unless 
there  be  chemical  agencies  at  work  in  the  sun  of  a  far  more 
powerful  order  than  anything  we  meet  with  on  the  earth's 
surface."  Failing  any  such  agency  Tait  proceeded  to  fall  back 
on  gravitational  energy  as  the  source  of  the  sun's  heat. 
Adopting  the  nebular  "hypothesis  of  Laplace,  that  the  various 
planets  and  suns  are  formed  by  the  falling  together  of  small 
masses  originally  attracted  to  one  another  from  great  distances, 
it  was  pointed  out  that  the  heat  evolved  on  the  impact  of  such 
masses  must  be  of  a  far  higher  order  than  the  same  masses 
could  produce  by  chemical  combination,  and  that  there  was 
no  difficulty  in  accounting  for  very  long  periods  of  solar 
radiation  on  this  hypothesis.  "But,  on  the  very  highest 
computation  that  can  be  permitted,  it  cannot  have  supplied 
the  earth  [during  the  past],  even  at  the  present  rate,  for  more 
than  about  15  or  20  million  years." 

The  almost  prophetic  qualification  with  regard  to  "  chemical 
agencies  of  a  far  more  powerful  order  than  anything  we  meet 
with  "  will  be  appreciated  at  the  present  time.  It  remains  to 
be  seen  whether  the  meteoric  or  any  similar  more  recent 
hypothesis,  such  as  that  of  shrinkage,  is  not  rendered  unneces- 
sary, or  at  least  of  subsidiary  importance,  by  the  existence  of 
the  available  energy  of  atomic  structures  undergoing  degrada- 
tion. Undoubtedly  radio-activity  is  but  one  manifestation  of 
such  energy.  If  all  matter,  on  the  average,  were  disinte- 
grating at  the  same  rate  as  uranium  without  exhibiting  radio- 
activity the  effects  would  have  escaped  observation  in  the 
laboratory  but  would  still  be  great  enough  to  be  the  ultimate 
controlling  factor  of  cosmical  evolution.  It  is  to  be  hoped 
that  the  whole  question  will  be  gone  into  exhaustively  in  its 
various  bearings  by  those  able  to  speak  with  authority  in  their 
respective  sciences. 

With  regard  to  radio-activity,  an  independent  limit  of  the 
past  age  of  the  earth  is  set  by  our  present  ignorance  of  any 
concomitant  process  of  atomic  reconstruction.  If  it  be  assumed 


186  RADIO-ACTIVITY. 

that  no  such  process  has  been  going  on,  at  least  for  the  last 
thousand  or  ten  thousand  million  years,  the  past  age  of  the 
radio-active  minerals  is  fixed  simply  by  the  period  of  the 
average  life  of  the  elements  uranium  and  thorium.  We  have 
seen  that  it  is  probable  that  all  the  other  cases  of  radio-active 
elements  will  ultimately  come  to  be  regarded  as  products  of 
more  slow-changing  elements,  such  as  uranium  and  thorium. 
From  this  point  of  view  the  minerals  containing  a  large  pro- 
portion of  uranium  must  have  been  formed  within  the  period 
of  the  average  life  of  the  atom  of  this  element.  The  value 
given  for  the  latter — 109  years — is  somewhat  uncertain,  and  a 
margin  as  between  108  and  109  years  should  be  allowed.  We 
thus  see  that  the  pitchblendes  and  uraninites  must  have  been 
formed  within,  say,  the  last  thousand  million  years,  and  possibly 
within  the  last  hundred  million  years.  With  regard  to  the 
age  of  the  earth,  some  further  information  is  obtained  by 
finding  the  proportion  of  uranium  remaining  at  various 
intervals,  assuming  108  years  as  the  period  of  average  life.  In 

108  years  the  quantity  would  be  reduced  to  -  in  2x  108  years 

1  ^ 

to  —  and  so  on ;  so  that  in  10°  years  the  total  quantity  would 

£ 

be  reduced  to  0-004  per  cent,  of  the  original.  In  1010  years  it 
would  be  reduced  to  4xlO~60.  So  that,  even  if  the  whole 
earth  were  originally  uranium,  and  no  reproduction  has  taken 
place,  it  cannot  have  survived  more  than  109  or  1010  years. 

The  Possibility  of  the  Reconstruction  of  Matter. — The  question, 
therefore,  naturally  arises  as  to  whether  this  really  sets 
the  limits  of  the  earth's  history — or,  indeed,  of  that  of 
any  body  in  which  uranium  is  present ;  or  whether  it  is  not 
more  philosophical  to  suppose  that  even  the  parent  radio- 
elements  are  being  reconstructed.  In  this  supposition  we  are 
met  at  once  with  what  appears  to  be  an  almost  insurmountable 
difficulty.  We  know,  from  the  energy  emitted  by  a  disinte. 
grating  atom,  the  absorption  that  must  take  place  if  that  atom 
is  to  be  reconstructed  out  of  its  constituents,  and  this  is  so 
great  that  it  is  difficult  to  imagine  how  it  can  be  furnished 
from  its  surroundings  unless  a  considerable  area  is  taxed  for 
the  supply.  At  first  sight  it  seems  that  the  atomic  theory, 
which  bears  out  and  is  borne  out  so  strikingly  by  atomic  dis- 
integration, opposes  a  barrier  to  any  conception  of  atomic 


ANTICIPATIONS.  187 

up-building.  A  gradual  and  continuous  accretion  of  atomic 
mass,  in  which  the  energy  was  supplied  in  very  small  steps  as 
available  from  the  surroundings,  seems  the  only  process  readily 
•conceivable.  But  the  atomic  theory  appears  to  demand  equally 
with  a  per  saltum  degradation  a  per  saltum  accretion.  This 
difficulty  is,  however,  not  real.  We  have  only  to  account  for 
the  step-by-step  increase  in  the  complexity  of  atoms  as  known 
to  us.  It  is  not  necessary  to  assume  that  it  is  impossible  for 
intermediate  forms  (representing  a  practically  continuous 
increase  of  atomic  mass  from  the  lightest  to  the  heaviest  known 
atom)  to  be  altogether  incapable  of  existence.  All  that  is 
required  is  that  the  rate  of  accretion  of  mass  should  be  more 
rapid  between  the  points  of  stability  as  represented  by  the 
Atoms  of  the  Periodic  Table.  This  would  ensure  that  the 
intermediate  forms  in  the  up-building  process,  like  the  transi- 
tion— forms  in  that  of  disintegration,  would  never  accumulate  in 
sufficient  quantity  to  have  been  yet  detected  by  direct  methods. 
The  only  question,  therefore,  is  as  to  the  source  of  the 
necessary  energy.  We  have  seen  that,  with  the  advent  of  the 
possibility  of  atomic  changes,  the  applicability  of  the  long- 
accepted  generalisations  of  science — the  conservation  of  mass, 
the  equivalence  of  electric  charges,  &c. — to  the  new  conditions 
-are  legitimate  objects  of  inquiry.  The  principle  that  is  now 
called  into  question  is  the  second  law  of  thermo-dynamics. 
This  undoubtedly,  with  the  others,  must  be  put  upon  its  trial, 
and  its  claim  to  universal  application  be  tested  anew.  Fortu- 
nately, owing  to  the  genius  of  Clerk-Maxwell,  the  limitations 
of  this  law  have  long  been  clearly  recognised.  From  the 
kinetic  theory  of  gases  it  follows  that  in  a  gas  of  uniform 
temperature  the  individual  molecules  must  be  moving  at 
varying  velocities,  and  therefore  possess  different  kinetic 
energies.  The  temperature  of  the  gas  represents  the  average 
temperature,  as  measured  by  the  kinetic  energy,  of  the  mole- 
cules. Clerk-Maxwell  imagined  some  supernatural  agency  to 
sort  the  molecules  into  two  parts,  the  one  possessing  kinetic 
energy  higher  than,  and  the  other  lower  than,  the  average. 
Such  a  process  can  be  imagined  to  be  effected  without  the 
performance  of  work,  and  the  gas  can  therefore  be  obtained  in 
two  portions  at  different  temperatures.  The  second  law  of 
thermodynamics  states  that  such  a  process  as  this  is  impossible 


188  RADIO-ACTIVITY. 

without  the  performance  of  work.  One  part  of  a  body  of 
uniform  temperature  cannot  grow  hotter  at  the  expense  of 
the  heat  of  the  remainder  unless  work  is  performed  upon  it, 
Yet,  according  to  the  kinetic  theory  of  gases,  a  gas  in  which 
all  the  molecules  were  originally  at  the  same  temperature, 
would,  after  a  short  time,  consist  of  molecules  at  different  tem- 
peratures, the  average  temperature  only  remaining  unchanged. 

The  question  therefore  arises,  Can  the  second  law  of  thermo- 
dynamics be  applied  to  sub-atomic  change  1  Or,  has  the  law, 
like  that  of  the  immutability  of  the  elements,  only  a  limited 
range  ?  All  processes  of  evolution  at  present  revealed  proceed 
in  the  one  direction,  the  energy  being  "  degraded  "  into  heat 
of  uniform  temperature,  which  is  no  longer  available  for  any 
useful  purpose.  A  limit  is  thus  fixed  when  all  change  must  cease 
unless  there  is  an  upward  process  going  on  in  Nature,  whereby 
the  energy  passes  back  again  into  available  forms.  This  is  the 
real  question  underlying  the  possibility  of  a  reconstruction  of 
the  elements  with  the  absorption  of  energy.  The  latter  is  so 
enormous  that  it  is  difficult  to  imagine  any  other  source  than 
heat  of  uniform  temperature,  to  be  sufficient  for  the  purpose. 

The  position  reached  is  at  least  definite.  If  the  doctrine  of 
the  transformation  of  energy  proceeding  spontaneously  in  only 
one  direction  is  universally  true,  the  limits  of  the  possible  age 
of  the  earth  would  seem  to  be  fixed  at  a  maximum  of  109  or 
1010  years.  The  end  of  evolution  is  definitely  fixed  as  occurring 
when  all  the  available  energy  shall  have  run  its  course  to 
exhaustion.  Correspondingly,  a  sudden  beginning  of  the 
universe — the  time  when  present  laws  began  to  operate — is  also 
fixed.  It  is  necessary  to  suppose  that  the  universe,  as  a  thing 
in  being,  had  its  origin  in  some  initial  creative  act,  in  which  a 
certain  amount  of  energy  was  conferred  upon  it  sufficient  to- 
keep  it  in  being  for  some  period  of  years.  It  is  possible,  it  is 
true,  to  avoid  the  end  indefinitely,  since  the  rate  of  change  will 
diminish  as  the  end  is  approached,  and  theoretically  the  end 
will  require  an  infinite  time  to  be  attained.  But  the  difficulty 
connected  with  the  beginning  cannot  be  so  avoided. 

The  alternative  view,  which  is  only  beginning  to  appear 
even  possible,  is  that  the  second  law  with  regard  to  the 
availability  of  energy  does  not  universally  apply,  and  that,  in, 
the  infinitely  varied  processes  of  Nature  a  cyclic  scheme  of 


ANTICIPATIONS.  189 

evolution  is  possible.  The  heavy  elements,  on  one  side  of  the 
cycle,  may  yield  a  continuous  supply  of  available  energy, 
while  the  lighter  elements  are  continuously  growing,  by  the 
gradual  accretion  of  masses,,  possibly  of  electronic  dimensions, 
and  at  the  same  time  storing  up  the  "  waste  "  energy  produced 
in  the  opposite  process.  The  essential  law  of  the  universe  may 
be  that  the  total  quantity  of  energy  is  constant,  and  that  what 
we  call  low-grade  and  high-grade  energy  is  the  expression  of 
the  limited  means  at  our  disposal  for  utilising  it.  If  we  lived 
in  a  sub-atomic  instead  of  a  molecular  world  possibly  the 
significance  of  the  terms  might  be  reversed.  The  universe 
would  then  appear  as  a  conservative  system,  limited  with 
reference  neither  to  the  future  nor  the  past,  and  demanding 
neither  an  initial  creative  act  to  start  it  nor  a  final  state  of 
exhaustion  as  its  necessary  termination.  This  point  of  view 
is,  in  every  way,  such  a  much  more  complete  and  satisfactory 
one  that  it  is  perhaps  necessary  again  to  emphasise  that  it  is 
only  at  present  a  mere  speculation,  or  perhaps  one  of  those 
coming  events  which  cast  their  shadows  before  !  The  really 
essential  advance  that  the  results  dealt  with  in  this  book  lead 
to,  is  that  the  limitations  with  respect  to  the  past  and  future 
history  of  the  universe  have  been  enormously  extended. 
Exactly  how  much  time  can  be  given  to  the  biologist  in  which 
to  work  out  his  processes  is  a  matter  which  will  require  some 
considerable  discussion  to  adjudge.  The  unconscious  appli- 
cation of  generalisations  which  have  only  a  limited  scope  to 
the  universal  problems  of  evolution  has  served  in  the  past  to 
restrain  the  legitimate  aspirations  of  science.  Out  of  the  revolu- 
tion, which  the  new  knowledge  must  effect  in  every  branch 
of  philosophy,  one  thought,  long  nascent,  at  length  begins  to 
take  definite  shape.  Each  new  advance  increases  the  period  of 
time  over  which  the  laws  of  Nature  can  be  regarded  as  having 
been  in  continuous  operation  without  external  interference. 

The  limitations,  with  respect  to  a  beginning  and  an  end, 
which  one  era  imposes,  disappear  in  the  next.  So  that  it  is 
not  unreasonable  to  anticipate  that  ultimately  these  laws  will 
be  recognised  to  operate,  not  only  universally  with  regard  to 
space,  which  has  long  been  admitted,  but  also  consistently 
with  reference  to  time. 


INDEX  TO   CONTENTS. 


INDEX  TO  CONTENTS. 


[Figures  in  Italics  denote  main  references.] 

PAGE 

a  Particle,  Mass  and  Velocity  of  13,  79,  80 

Suggestion  that  it  io  an  Atom  of  Helium. .         . .        163,  182 

a  Particles,  Mass  of,  expelled  per  Gramme  of  Kadium  per  Hour    . .     168 

Number  of,  produced  at  each  Disintegration     . .        112,  169 

a  Radiation  accompanies  Change          . .          . .         . .         . .         . .     112 

a-Rays       . .        12,  27,  29.  32,  46,  58,  65,  75  et  seq.,  85,  88,  101,  109,  130, 
133,  135-139,  146,  163,  166,  168,  175,  181,  182 
Absorption  of,  by  Matter  . .  12,  27,  65,  75  et  seq.,  163 


Action  on  Zinc  Sulphide 

are  the  most  important  Type 


Chemical  Action  of 


Deviation  of,  by  Magnetic  and  Electric  Field. .       13,  67,  77  et  seq. 


Fluorescent  Actions  of 

Heating  Effect  of 


27,  58,  80 
. .  75,  136,  167 
27,  128 


27,  28,  58,  80 
. .  27,  166,  168 


Non-separable  Activity  consists  only  of     88,  95,  96,  113,  133, 137 
—  of  Polonium         29,  76 

Kadium,  27,  71,  75,  lletseq.,  130, 133, 136  et  seq.,  146, 163, 166, 

182 

Thorium          15,  66,  75,  88,  95,  148 

Uranium         15,  65,  75,  85,  88,  96,  146 

Effect  on  Photographic  Plate  of  . .  57,  85 


Penetrating  Power  of 12,  65,  75 

Positive  Charge  carried  by        13,  46,  80,  181,  182 

Radiation  of  Radium  Emanation  consists  only  of    . .        136,  150 

Thorium  Emanation  consists  only  of  . .         . .     101 

Similarity  of,  for  all  Radio- Active  Substances          . .         . .       80 

Abnormalities  caused  by  Subsequent  Changes          . .         . .  90,  117,  134 
Absorption,  Effect  of,  on  Measurements  of  Radio- Activity. .        16,  20,  63 

of  Radium  Emanation  by  Celluloid         130 

Rays  by  Gases 57,  58,  71,  76 

proportional  to  lonisation      ..  38,58 

Matter   . .     9,  10,  12,  13,  27,  28,  58,  63,  65  et  seq. 

X-Rays  by  Vapours         68,  b9 

Acceleration  of  Corpuscle  the  cause  of  X-Rays  . .         . .        8,  69,  7C 

Accompaniment  of  Change,  Radiation  the       . .         . .         •  •          . .     112 

Radio-Activity 


194  RA  DIO-A  CTI VITY. 

PAGE 

Accretion  of  Atomic  Mass  by  Electrons  . .          . .          . .         186,  187 

Accumulation  of  Emanation  in  Non-Emanating  compounds         120,  131 

Ultimate  Products 116,158 

Actinium 22,  30,  31,  33,  140,  141,  144,  176 

Discovery  of 30 

Emanation 31,140 

Charge  carried  by  . .         . .         . .         . .     140 

Evidence  of  Complexity  of  the  Changes  in  . .         .  -     141 

Badiations  of 31 

Separation  from  Eadium 22 

Test  for  Presence  of   . .         . .  144 

Actual  Life  of  Atom  Indefinite 125,  177  et  seq. 

Adams  :  Flow  of  Crystalline  Hocks  under  Pressure  . .         . .         . .     184 

Additive  Property,  Eadio-Activity  an 20 

^Eschynite,  Radio- Activity  of  . .          . .          . .         . .          . .       16 

Age  of  the  Earth 116,  172,  182,  186 

Eadio-Active  Estimate  of      ..         ..         ..         ..     186 

Air,  a  Ncn-conductor  of  Electricity 11,38 

Alcohol,  Condensation  of,  on  Ions        ..          ..         ..         ..          ..       45 

Allen,  H.  S.  :  Eadio-Active  Gas  in  Tap  Water  . .          . .          . .     158 

Ammonia,  Separation  of  Thorium  X  from  Thorium  by        . .          . .       86 

Analysis  of  Eadio-Activity  of  Eadium 133,  137,  146 

Thorium  . .          . .     88,  106,  118,  146 

-  Uranium  85,  95,  146 

—  Eadio-Active  Methods  of  Chemical  . .          . .  21,  33,  142  et.  seq 
Anomalous  lonisation  of  Heavy  Vapours  by  X-Rays  . .  68,  69 

Hydrogen 59,  68 

Anti-cathode,  Purpose  of  . .         . .         . .          . .          . .          . .         7 

Anticipations         . .         . .          . .         . .          . .          . .          . .     171  et  seq. 

Apparatus  for  Electrical  Measurement  of  Eadio-Activity     . .      58  et.  seq. 

for  showing  Condensation  of  the  Emanations     . .    151  et.  seq. 

of  Strutt  for  showing  Self-Electrification  of  Eadium       74,  181 

Argon,  Emanations  allied  to,  in  Chemical  Properties      103,  117,  130,  150 
Atmosphere,  Effect  of  Nature  of,  on  the  Proportion  of  Emanation 

Condensing         . .          . .          . .          . .          . .          . .      155 

Nature  of,  without  Effect  on  Emanations          . .         100,  104 

Eare  Gases  of          ..          ..          ..          ..         ..         ..     117 

Atom,  Disintegrating       ..          ..          ..          ...         ..          ..          ..     115 

Nature  of ....       171,176 

Atomic  Charge       . .          . .          . .          . .         . .         . .          . .  44,  79 

Disintegration,  due  to  Chance  Collocations  of  the  parts    . .     179 

Theory  of      55,  82,  89,  94,  109  et.  seq.,  121  et  seq., 

127,  138,  155,  167,  170 

-  Property,  Eadio-Activity  an 17,  20,  81,  82,  122 

Eeconstruction    . .         . .         . .         . .          . .       185,  186  et.  seq. 

Structure 54,  93,  122,  125,  178  et.  seq. 

Internal  Energy  of     ..          ..         34,122,164,169,170 

-  Theory  of  Dalton  ..         ..         ..         ..     55,112,177,186 

Weight,  a  Function  of  Internal  Energy         . .          . .        164,  180 

—  of  Eadium         23,  26 

-    Discrepancy    between    Chemical    and 

Spectroscopic  Evidence       . .          . .       26 

Weights,  Numerical  Eelations  between          . .          . .          . .     164 

Atoms,  Individual  Differences  between  . .          . .          . .         125,  177 

Internal  Motion  of          54,  178,  180 

Irregularity  of  Internal  Motion  of 180 

Number  of,  in  1  cubic  cm.  of  Hydrogen        . .         . .         . .       45 


IXDEX  TO   CONTENTS.  195 

PAGE 

Autunite,  Kadio- Activity  of        16 

Average  Life  of  Atom 125,147,177 

Radium  Atom  168  et  seq. 

Uranium  and  Thorium  186 

Lives,  Table  of 147 

Properties,  Chemical  Properties  must  be      . .         . .        125,  178 

Temperature  of  a  Gas . .         . .         . .  . .         . .     188 

Imparted  Activity         . .         . .         . .         . .         . .         . .     141 

ft  Particle,  Identity  with  Corpuscle 54,72,80 

Increase  of  Mass  with  Velocity  of 54,  74,  180 

-Mass  of  72,74,180 

Value  of  e/m  for  72,74 

Velocity  of        54,  72 

£-Rays,  12,  13,  15,  27,  29,  32,  46,  54,  57,  58,  59,  65,  68,  70  et  seq.,  85, 
88,  94,  100,  101,  109,  134,  135,  137,  146,  150,  175,  180,  181 

absent  in  Non-Separable  Activity        . .    88,  95,  96,  113,  133,  137 

Eadiation  of  Polonium        . .         . .          29,  32,  65,  175 

Radiations  of  the  Emanations       ..    101,134,136,150 

Absorption  of,  by  Matter  . .         . .  12,  27,  32,  66,  68,  70,  181 

Connection  between  7-Rays  and          . .         . .         . .      13,  69,  70 

with  Cathode  Ray  . .         . .     13,  67,  70,  72,  80 


Deviation  of,  by  Magnetic  and  Electric  Field  13,  67,  70  et  seq. 
Effect  of,  on  Photographic  Plate  . .  . .  13,  15,  57,  70 
Fluorescence  caused  by  .. 27,58,150 


Negative  Charge  carried  by 
of  Radium 


Thorium 
Uranium 


Penetrating  Power  of 


..13,  27,  46,  72,  73,  74,  181 
27,  70  et  seq.,  135,  137,  146 

. .      15,  66,  70,  88,  135,  146 

15,  66,  70,  85,  94,  135,  146 

. .     12,  27,  32,  66,  70 


Relatively  Small  lonation  caused  by  . .         . .         13,  66,  75,  100 

Result  in  Last  Stage  of  Disintegratin  . .         . .        135,  146 

Similarity  to  7-Rays      ..         ..         ..      -   ..    *     ..         ..       68 

Barium,  Radio- Active 17,84,87 

Barker  :  Complex  Nature  of  Thorium ..         ..       96 

Baskerville  :  Complex  Nature  of  Thorium 96 

Becquerel,  Henri :  Deviability  of  a-Ray  of  Radium 79 

, /3-Ray  by  Magnetic  Field  . .  70,  72 

Discovery  of  Radio-Activity  . .         . .         . .          . .         . .  2,  10 

Electrical  Conduction  of  Gases  under  Uranium  Rays     . .       37 

Enfeeblement  of  Radio-Activity  of  Uranium  by  Chemical 

Treatment 84,  96 

Nature  of  /3-Rays         13,  70,  72 

Use  of  Photographic  Method  

Biology,  Bearing  of  Radio-Activity  on 183,  189 

Bismuth,  Radio-Active 17 

Brauner  :  Complex  Nature  of  Thorium  96 

Brooks,  Miss,  and  Rutherford  :  Characteristics  of  Rays  from  various 

Substances    . .         . .         . .         . .         . .         . .  31,  75 

Complexity  of  Changes  causing  Imparted  Activity          . .     138 

Diffusion  Coefficient  of  Radium  Emanation         . .         . .     157 

Bumstead  and  Wheeler :  Diffusion  Coefficient  of  Radium  Emanation     158 

Presence  of  Radium  Emanation  in  Soil  and  Water        . .     158 

Bunsen  and  Kirchoff :  Discovery  of  Caesium..         ..         ..         ..       19 

Caesium,  Discovery  of     . .          . .         . .         . .         •  •         •  •         •  •       19 

€apacities,  Use  of,  in  Radio-Active  Measurement 

o2 


196  RADIO-ACTIVITY. 

PAGE 

Carnotite,  Eadio-Activity  of       . .         . .         . .         . .         . .         . .       16 

Cathode  Kay  Particle,  Mass  of 53,  54 

Value  of  elm  and  v  for  . .  . .  50,  51 

Bays 5-10,  13,  38,  46,  50-54 

Absorption  of,  by  Matter  9 

Connection  with  /3-Bays  ..  ..  13,  67,  72,  80 

the  Negative  Ion  . .  . .  46,  52 

X-Eays  7,10 


-  Deviation  of,  by  Magnetic  and  Electric  Field  10,  50 

—  Direct  Determination  of  Velocity  of 51 

Fluorescence  caused  by ..         ..         ..         ..          ..6,10 

lonisation  of  Gases  by   * 38,  46,  51 

—  Penetrating  Power  of    . .         . .          . .          . .         . .    8,  9 

Eesemblances  between  X-Eays  and    . .          . .        9,  10,  14 


Cause  of  Eadio-Activity  unknown         122,179 

Celluloid,  Absorption  of  Kadium  Emanation  by         130 

Chalcolite,  Eadio-Activity  of 16 

Chance  Collocation  of  Parts,  Atomic  Disintegration  due  to. .         . .     179 
Change,  a  Eadiation  accompanies         112 

Eadio-Active         35,  89,  90,  105,  107,  111  et  seq.,  146,  158  et  ?eq. 

Effect  of  Conditions  on  . .         . .      93,  103,  134, 177 

Energy  of 91,  128,  163  et  seq.,  183 

Law  of 91,114,122,176 

Changes  of  Eadio-Activity  due  to  Alteration  of  Number  of  Eays 

expelled  in  Unit  Time         110 

Bayless *..         ..        139,141 

Charge,  Atomic 44,  79 

carried  by  a-Eay 13,46,80,181,182 

/3-Kay 13,27,46,72,73,74,181 

-  the  Emanations 101,141,181 

Charges,  Law  of  Equivalence  of  Electric         171,180 

Chemical  Actions  of  Eadium  Eays        . .         . .          . .         . .          27,  128 

Analysis  of  the  Eadiations  of  Uranium      . .         . .         . .       85 

Energy  inadequate  to  maintain  Solar  Eadiation  . .         . .     185 

Nature  of  the  Emanations ,  103,  130,  349 

Imparted  Activity  Matter     . .  105,  106,  157 

Properties  must  be  Average  Properties       . .          . .         125,  178 

Clerk-Maxwell :  Electromagnetic  Theory  of  Light    . .          . .          . .         3 

Second  Law  of  Thermodynamics 1 87 

Sorting  Demon  . .         . .         . .          . .          . .          . .     187 

Cleveite,  Eadio-Activity  of          16 

Cluster  carried  by  Ions     . .         . .         . .         . .          . .          . .  45,  46 

Combination,  Eadio-Active  Change  not  a        115 

Complex  Nature  of  Thorium      . .         . .         . .         . .         . .         . .       96 

Complexity  of  /3  Eadiations  of  Eadium  72 

Changes  causing  Imparted  Activity    . .         . .         138,  139 

Eadiations  from  Uranium        . .          . .          . .          . .       65 

Eadio-Active  Phenomena          . .          . .      73,  117,  123,  138 

Concentration,  Effect  of,  on  Eate  ofiCondensation  of  the  Emanations    156 

Independence  of  Eadio-Active  Change  on     ..        115,379 

of  Imparted  Activity  on  Negative  Electrode  101,  106,  111, 

132  135,  141,  181 


Condensation  of  Alcohol  and  Ether  and  Ions  . . 


Emanations 104,130 


. Water  Vapour  on  Ions 

Conditions,  Effect  of,  on  Eate  of  Escape  of  Emanations 


—  Independence  of  Eadio-Active  Change  on 


45 

150,  152,  157 

..       44 

120,  131 

.  93,  103,  134 


INDEX  TO   CONTENTS.  197 

Conduction  Leak,  Distinction  between  lonisation  Leak  and  63 


of  Electricity  through  Gases  37  et  seq. 

Connection  between  Eadiation  and  Change    ..         ..         ..     109  et  seq. 

Conservation  of  Mass,  Law  of,  applied  to  Radio-Active  Change  164,  180 

Radio-Activity  , 115,  138 

Conservative  System,  Possibility  that  the  Universe  is  a  . .  . .  189 
Consistency  of  Natural  Law  with  Eespect  to  Past  and  Future  Time  189 

Constancy  of -Radio-Activity 34,84,93,115,172 

Constant  Rate  of  Production  of  Thorium  X 93,  112 

The  Radio-Active  92,113.116,125,168 

Constants,  Table  of  the  Radio-Active 146 

Continuous  Production  of  Thorium  X 89,  92 

Controlling  Factor  in  Radio- Active  Change 176 

Convection  of  Ions,  Effect  of  Electric  Field  on  48 

Cornish  Pitchblende,  Radio-Activity  of  16, 21 

Corpuscles..  ,..  ..-  .. 53,55 

Acceleration  of,  the  Cause  of  X-Rays  . .  . .  8,  69,  70 

Expelled  by  Radium  with  Velocity  approaching  that  of 

Light  54, 74,  80 

•  Identity  with  £-Ray  Particle  54,  72,  80 

Increase  of  Mass  with  Velocity  of 54,  74,  180 

Corpuscular  Theory  of  Electricity  53,  80 

• Light  ..  2,14 

Types  of  Radiation 5,  14,  70,  77 

Cosmical  Evolution 172,  188 

Creative  Act,  The  Necessity  of  an  Initial        188 

Critical  Velocity  of  Ion 47 

Crookes  :  Discovery  of  Cathode  Rays  . .          . .         . .          . .         . .         2 

UraniumX ..       83 

Radiant  Matter          5 

•  Radio-Activity  of  Uranium 83,  96 

Scintillations  of  Zinc  Sulphide       27,  80 

.  Spinthariscope  27,  80 

Crookes  and  Dewar :  Spectrum  of  Fluorescent  Light  of  Radium  . .     160 

Curie  :  ^-Radiation  of  Radium 136 

Non-Deviability  of  Easily-absorbed  Radium  Ray      . .         . .       70 

Rate  of  Decay  of  Radium  Emanation. .         . .  .         . .     134 


Curie,  Mme. :  Absorption  of  ci-Rays  of  Polonium 
Atomic  Weight  of  Radium 


Diagram  of  the  Deviability  of  a,  j8  and  7-Rays 
Discovery  of  Radio-Activity  of  Thorium 


76 
23 
67 
15 


Induced  Activity  of  Radium  with  Slow  Rate  of  Dissipa- 
tion             140,  174 

Purification  of  Radium  Compounds  22 

Radio-Activity  an  Atomic  Property  . .         . .      17,  20,  82 

of  Artificial  Chalcolite        16 

Minerals  .  . .         . .         •  •         •  •       16 


Separation  of  Polonium  . .  . .  . .  •  •  • '  28 

View  that  the  Radio-Active  Atom  is  a  Continuous  Source 

of  Energy     . .         . .         . .          . .         . .         •  •         •  •       $2 

Curie,  M.  and  Mme. :  Apparatus  for  Electrical  Measurement  of 

Radio-Activity         60 

Discovery  of  Radium  and  Polonium          . .         . .         . .  17, 18 

Induced  Activity        33,84,132 

Negative  Charge  carried  by  £-Ray  of  Radium         ..         ..       72 

Curie  and  Danne  :  Complexity  of  Changes  causing  Imparted  Activity 

Diffusion  Coefficient  of  Radium  Emanation      . .         . .     156 


198  RADIO-ACTIVITY 

PAGE 

Curie  and  Dewar  :  Production  of  Helium  from  Kadium      . .         . .     160 
Curie  and  Laborde  :  Energy  emitted  by  Kadium  Compounds         . .     165 

Current,  Saturation          39,  42,  58,  60,  f3 

through  Gas  produced  by  Thorium    ..         ..         ..         ••       60 

Cyclic  Evolution,  Possibility  of 172,  188 

Dalton's  Atomic  Theory 55,  112,  177,  186 

Debierne  :  Charge  carried  by  Actinium  Emanation  ..         ..  .     141 

Discovery  of  Actinium  . .          . .         . .         . .  .       30 

Emanation  and  Imparted  Activity  of  Actinium   . .        140,  141 


Enfeeblement  of  Radio-Activity  of  Uranium 
Extraction  of  Eadium  from  Pitchblende 
Imparted  Activity  of  Eadium 


84 
30 
132 


Decay  of  Activity  of  Actinium  Emanation     . .         . .         . .  32,  140 

Imparted  Activity      ..          ..          ..     140 

Polonium 29,  30,  113,  173,  174 

Radium  Emanation        32,  130,  132 

Imparted  Activity         . .  132,  136,  139 

(|8  Radiation)       ..     ]36 


Thorium  Emanation       . .         . .     32,  101,  103,  104 
Imparted  Activity      . .         . .        105,  138 


Thorium  X  . .         . .       87,  88,  90,  92,  117,  119 

Uranium  X  85,95 

Physical  Meaning  of  ..          ..         ..  92,113 

Emanating  Power  of  Thorium  X 87,  88,  110 

Decomposition  of  Water  by  Radium  Rays 27,  128 

De-emanation        119,120 

Demarcay  :  Spectrum  of  Actinium 30 

Radium         17,  24 

Density,  lonisation  of  Gases  Proportional  to  Density  . .  38,  58 

Law  of  Absorption,  Exceptions  to  . .          . .  68,  76 

for  a,  j3  and  7-Rays         . .         . .      13,  68,  76 

Uranium  Rays  . .          . .          . .       11 

X-Rays  9,63,69 

of  Compounds,  Effect  on  Radio-Activity       . .          . .      16,  20,  63 

Des  Condres  :  Value  of  elm  for  a  Particle        . .          . .          . .         . .       79 

Deslandres  :  Spectrum  of  Helium  from  Tube  containing  Radium  .     ItiO 
Destruction  of  Radio- Activity  impossible         . .          . .          . .          . .     105 

Deviation  of  ct-Rays  by  Magnetic  and  Electric  Field. .      13,  67,  77  et  seq. 

/3-Rays  by  Magnetic  and  Electric  Field  . .       13,  67,  70  et  stq. 

Cathode-Rajs  by  Magnetic  and  Electric  Field  . .          . .  10,  50 

Radiant  Ion  by  Magnetic  and  Electric  Field      . .          . .       49 

Dewar  and  Crookes  :  Spectrum  of  Fluorescent  Light  of  Radium  . .  160 
Dewar  and  Curie  :  Production  of  Helium  from  Radium  . .  . .  J60 
Diamond,  Fluorescence  of  Under  o-Rays  of  Radium  . .  . .  58 

Differences  between  Individual  Atoms  125,  177 

in  the  Condensation  of  Thorium  and  Radium  Emanations     154 

Diffusion  Coefficients  of  Ions  in  Gases . .         . .  .          . .          . .       43 


of  Radium  Emanation 


of  the  Emanations 


Diminution  of  Volume  of  Radium  Emanation  with 
Direction  of  Deviation  of  a-Ray 
Discharge  of  Electricity  through  Gases 
Rarefied  Gases 


157,  168 
105,  157,  175 
Time  ..          ..162 
. .      13,  67,  79 
36  et  seq. 
49  et  seq. 
80 


Discrete  Nature  of  a  Radiation 

Disintegrating  Atom         115 

Disintegration,  Energy  of  . .          . .         . .          . .          . .        167,  169 


INDEX  TO   CONTENTS.  199 

Disintegration  Series,  Table  of 146 

Theoretical  consideration  of  . .         . .         . .     172 

Theory,   55, 82, 89, 94, 109  et  seq.,  121  et  seq.,  127, 138, 155 

167,  170 

Dissimilarity  of  Successive  Products  of  Radio-  Active  Change         . .     107 
Distinction  between  lonisation  Leak  and  Conduction  Leak. .         . .       6$ 

Ions  and  Emanations      . .         . .         . .         . .       39 

Disturbances  caused  by  Subsequent  Changes..         ..         ..90,  117,  134 

Precautions  against  Electrostatic         . .         . .         . .       61 

Dolezalek  Electrometer  . .         . .         . .         . .         . .         . .         . .       61 

Dorn  :  Discovery  of  Radium  Emanation         130 

Double  Change,  Imparted  Activity  of  Thorium  due  to  a      . .         . .     13S 
Dual  Nature  of  Radio-Elements  . .  27,  1«2,  149 

E-Ray  of  Actinium  142 

e  Charge  carried  by  Ions  Determination  of  . .         . .         . .       45 

is  the  Atomic  Charge         . .         . .         .  •      44 

—  Value  for  a  Particle 72 

/3  Particle 79 

Earth,  Age  of..  116,  172,  182  et  seq. 

Radio-active  Estimate  of          186 

A  Self -Heating  Planet        183 

Internal  Heat  of 183 

Electric  Charges,  Law  of  the  Equivalence  of  . .         . .       172,  180,  et  seq. 

Field,  Effect  of,  in  concentrating  the  Imparted  Activity  . .    101, 

106,  111,  132,  135,  141,  181 

Rlectrical  Mass  or  Inertia          54,  75,  80,  180 

Methods  of  measuring  Radio- Activity        . .  57,  58  et  seq. 

Properties  of  Gases 37  et.  se<^ 

Electricity,  Corpuscular  Theory  of 53,80 

Electromagnetic  Induction         . .         . .         . .          . .         . .         . .         4 

Oscillations      . .         . .         . .         . .         . .         . .         4 

Theory  of  Light         ,         . .         3 

Electrometer,  Dolezalek  . .         . .         . .         . .         . .         . .         . .       61 

Quadrant 59 

Electronic  Accretion  of  Atomic  Mass   . .          . .          ..         ..         •  •     189 

Radiation  from  Atom  179 

Theory  of  Matter 55 

Electrons 53,  55,  75, 189 

Electroscope,  Discharge  of,  by  Radium 

Uranium         . .         . .         . .         . .       H 

Gold-Leaf ..      11,61,78 

Precautions  to  be  observed  in  using 


Electrostatic  Deviation  of  a-Ray          . .         . .         -. .         • .         . .  67,  79 

0-Ray  67,72 

Cathode  Ray          50 

Radiant  Ion  49 

Disturbances,  Precautions  against      . .         . .         . .       61 

Elements,  The  Five  Radio- Active         15  et  teq. 

Unstable          

Radio-Activity  the  Science  of  the 

Elster  and  Geitel :  Existence  of  an  Emanation  in  Soils      ..         ••     158 

Scintillations    Produced    by    a-Rays    on    Zinc 

Sulphide  Screen  80 

Emanating  Power,  Connection  of  Radio-Activity  with         . .        110,  1J 
Emanation  Atoms,  Number  of,  produced  from  each  Radium  Atom    _  169 

not  Produced  by  Uranium 3 2,  9o,  1< 

of  Actinium     . .         . .         . .         . .         -  -      32,  101, 140,  144 


200  RADIO-ACTIVITY. 

PAGE 

Emanation  of  Eadium     . .         . .  28,  32,  130  et  seq.,  142,  149  et  seq. 

Properties  of 130,  149  et  seq. 

Proportion  of  Activity  due  to 137 

Presence  of,  in  Soil  and  Water  . .          19,  158 

Thorium      . .         . .  32,  86,  99  et  seq.,  143,  152  et  seq. 

Due  to  Thorium  X        86,  104 

Properties  of 100,  103 


Substance  of  Giesel 31,  142 

Emanations,  Eadio- Active,  19, 32,  89, 100  et  seq.,  130  et  seq.,  140,149  et  seq. 
-    Charge  carried  by  the       . .  101,  141,  181 

: Condensation  of  the  104,  130,  150  et  seq. 

Diffusion  of  the     ..         ..  105,157,175 

Distinction  between  Ions  and     . .         . .       39 

Energy  Association  with  Matter  a  Maximum  in  a  and  j3  Particles,  81,  123 

—  Emission  without  Absorption  necessitates  Material  Change       35 

—  emitted  by  Kadium        . .         . .         . .         . .         . .         . .     165 

i-r Uranium . .          . .     165 

—  necessary  for  Atomic  Reconstruction. .          ..          ..          ..     187 

,-r  of  Radiant  Ions  46 

Radio-Active  Change  ..         ..94,  128,  163,  et  seq.,  183 

Substances,  Source  of  33,  94,  121,  122,  167, 

182  et  seq. 

required  to  Produce  an  Ion  the  same  in  all  Gases    . .         . .       59 

Equation  Connecting  Velocity  and  Diffusion  Coefficient  of  Ions      . .       44 

of  Current  flowing  through  a  Gas  . .         . .          . .       41 

Equilibrium,  Radio-Active          93,  94,  115,  124 

Equivalence  of  Electric  Charges  in  Radio-Active  Change  172,  180  et  seq. 
Escape  of  Emanation,  Rate  of,  affected  by  Conditions         . .        120,  131 
Ether,  Condensation  of  Vapour  of,  on  Ions    . .         . .         . .         . .       45 

Mechanical  and  Electromagnetic  Theories  of  . .          . .         4 

Necessity  of  postulating  the  Existence  of  a  Universal        . .         3 

Eve,  A.  S. :  Normal  lonisation  of  Heavy  Vapours  by  Penetrating 

X-Rays  69 

Evolution,  Beginning  and  End  of         188 

of  Matter          123,  125,  126,  140,  189 

Possibility  of  a  Continuous  Cycle  of  Cosmical      . .         172, 188 

Excited  Activity  (see  Imparted  Activity)         33 

Explosion  of  Compounds  122,  123,  176 

Radio-Active  Atom  70,  122 

Sealed  Tubes  containing  Radium  74 

External  Energy  in  Relation  to  Radio -Activity          . .         . .    34.  94,  122 

Extraction  of  Actinium  from  Pitchblende 22,30,31 

r-. Polonium  from  Pitchblende     . .          . .          22,  28-30,  174 

Radium  from  Pitchblende         . .          . .  17,  21  et.  seq. 

Eye,  The  Effect  of  Radium  Rays  on  the          130 

Faraday :  Definition  of  the  Term  "  Ion  "         , 46 

Electromagnetic  Induction  . .          . .          . .         ....         4 

Fergusonite,  Radio- Activity  of  . .         . .         . .          . .          . .         > .       16 

Five  Radio-Elements,  Characteristics  of  the 31-33 

Flame  Coloration  of  Radium 23 

—  Spectrum  of  Radium      . .          . .         . .          . .          . .          . .       26 

Flow  of  Crystalline  Rocks  under  Pressure . .     184 

Fluorescence  caused  by  X-Rays  . .          . .          . .          . .          . .        7,  10,  58 

Methods  of  measuring  Radio-Activity  . .          . .          . .       58 

of  Barium  Platinocyanide  . .          . .          . .        7,  27,  58 

—  Kunzite,  Diamond,  &G.  . .          . .          . .          . .       58 


1XDEX   TO   CONTEXTS.  201 

PAGE 

Fluorescence  of  Zinc  Sulphide   . .         . .         . .         . .         . .      27,  30,  58 

Produced  by  Radium  Emanation  ..         ..          28,150 

the  Suggested  Cause  of  X-Rays 10 

Fluorescent  Light  of  Radium,  Spectrum  of     . .         . .          . .         . .     160 

Substances  changed  by  Prolonged  Action  of  Radium  Rays    129 

Focus  Tube,  The 7 

Fractionation  of  Radiferous  Salts         18 

7-Rays        12,  13,  28,  32,  68  et  seq. 

Absorption  by  Matter 13,  67,  68 

Connection  with  £-Rays 13,69,70 

Discovery  of,  for  Radium          . .         . .         . .         . .         . .       66 

Thorium  and  Uranium       . .         . .         . .       67 

Explanation  of 69,  70 

Fluorescence  caused  by  . .         . .         . .         . .         . .         . .       28 

—  Non-Deviability  of          13,  67,  68 

Penetrating  Power  of 13,  28,  68 

Produced  only  in  the  Last  Stage  of  Disintegration  . .          . .     136 

Relative  Insignificance  of         13,  67,  70 

Gadolinite,  Inactivity  of  Thorium  obtained  from       . .         . .  96,  97 

Gas,  Emanation  has  the  Properties  of  a          . .    100,  101,  105,  149  et  seq. 

Number  of  Molecules  in  a  cubic  centimetre  of  . .         . .         . .       45 

Gaseous  Ions,  Two  Classes  of    . .          . .         . .          «.          . .         . .       46 

Gases,  Absorption  of  Rays  by 38,  57,  58,  71,  76 

Electrical  Properties  of  37  et  seq. 

Gates,  Miss  :  Volatility  of  Matter  causing  Imparted  Activity           . .     157 
Geological  Epochs,  Accumulation  of  Products  during          . .          . .     116 
Geometrical  Progression,  Radio- Activity  Decays  according  to  a     . .     113 
Giesel :  Deviability  of  /3-P.ays 70 

—  Effect  of  Radium  Rays  on  the  Eye     . .  130 


—  Emanation  Substance 

—  Flame  Coloration  of  Radium 


Method  of  Purifying  Radium  from  Barium 

Movement  of  Actium  Emanation  in  Electric 


Production  of  Polonium  from  Radium 

Quantity  of  Radium  in  Pitchblende    . 

Glass  impervious  to  the  Thorium  Emanation 
Gold-Leaf  Electroscope  . . 


31,  141 
. .       23 

22 

Field  . .         . .     142 
. .     174 
. .       21 
. .     100 
11,  27,  61,  78 

Goldstein  :  Canal  Rays 53 

Gravitational  Energy,  Maintenance  of  Solar  Energy  by       . .         . .     185 

"Hard  Tube" 8 

Heat,  De-emanation  produced  by         . .         . .         . .         . .         •  •     119 

Evolution  caused  by  a-Rays        27,  166,  167 

—  of  Radium         27,  34,  165 

Emanation  . .          . .          . .         . .        166,  167 

of  Uranium 165 

of  Uniform  Temperature,  Availability  of,  for  the  Processes  of 

Atomic  Reconstruction  . .         . .          . .         .  •         . .     188 

Heaviest  Elements,  Radio- Activity  the  Property  of  the        . .         . .       24 

Heaviside  :  Increase  of  Electrical  Mass  with  Velocity          . .         . .       54 

Helium,  Prediction  with  regard  to  Connection  of,  with  Radio- Activity,  117 

Presence  of,  in  Glass  subjected  to  a-Rays 163 

1_  Minerals          117,163 

Production  of,  from  Radium  . .          . .  •  •        117.  158 

Suggestion  that  a  Particle  is  an  Atom  of 163 

Hertz  :  Electromagnetic  Oscillations    . . 


202  RADIO-ACTIVITY. 

PAGE 

High  Voltages  necessary  to  obtain  Saturation  Current  with  Kadium  63 
Hittorf :  Eesistance  of  Short  Gap  to  passage  of  Discharge  in 

Earefied  Gases  49 

Hoffmann  and  Zerban  :  Inactivity  of  Thorium  from  Gadolinite  . .  96 
Huggins,  Sir  William  and  Lady:  Spectrum  of  Light  emitted  by 

Kadium  ICO,  16 i 

Hydrogen,  Anomalous  lonisation  of 59,  68 

Atom,  Mass  of,  compared  to  a  Particle      . .          . .  13,  79 

Cathode  Eay  Particle        . .       53 

Excess   of,   Produced   in   Decomposition   of    Water    by 

EadiumEays         129 

Number  of  Molecules  in  a  cubic  centimetre  of  45 


Imparted  Activity,     33,  73,  75,  84,  101,  105,  120,  132,  138,  140,  143,  146 

Effect  of  Electric  Field  on,      101,  106,  111,  132,  135, 

141,  181 

Matter,  Chemical  Properties  of    . .         . .        105,  106 

Volatility  of  157 

of  Actinium  33,  140,  144,  146 

Eadium 33,  75,  132,  138,  142,  146 

Thorium   . .         33,  75,  101,  105,  120,  138,  143,  146 


Importance  of  a  Eadiations        . .         . .         . .         . .         . .          75,  13fr 

Impossibility  of  Destroying  Eadio- Activity      . .  . .         . .     105 

Increase  of  Activity  of  Eadium  Salts  with  Time        . .         . .  84,  133,  137 

with  Time,  Meaning  of    . .          . .          . .          . .     116 

Independence  of  Average  Life  on  Period  of  Past  Existence . .        178,  179- 

Eadio-Active  Change  on  External  Conditions,  93,  103, 

134,  177 
Individual  Differences  between  Atoms  of  the  same  Element        125,  177 

Induced  Activity  (see  Imparted  Activity)         33 

of  Eadium  with  Slow  Kate  of  Dissipation    140,  173,  175 

Induction,  Electromagnetic        4 

Hypothesis  of  Eadio-Active  . .         . .  84,  87,  89,  94,  132 

Inertia,  Electrical 54,75,80,164,180 

Inertness  of  the  Emanations  of  Eadium  and  Thorium  to  Chemical 

Eeagents         "..         ..  103,130,149 

Infinitesimal  Quantity  of  Transition-Forms,  28,  83,  87,  103,  106,  113, 

128,  156,  162 

Initial  Creative  Act          188 

Irregularities  due  to  Subsequent  Products  of  Change,  90, 95, 117, 134 

Insulators,  Transparent  Bodies  the  Best         .  

Internal  Energy,  Atomic  Weight  a  Function  of          . .         . .         164,  180 

in  Eelation  to  Sadie-Activity . .         . .       34,  94,  122,  186 

—  of  Atomic  Structure    . .          . .        34,  122,  164,  169,  1 70- 

of  Kadium  Atom          . .          . .          . .          . .          . .     1691 

Heat  of  Earth 183 

Motion  of  Atomic  Structure      ..          ...         ..          ..         178,179 

Ion,  Energy  required  to  Produce  an 591 

Ionic  Charge          . .          . .         . .          . .          . .          . .          . .          . .       44 

Velocities 43- 

lonisation  and  Conduction  Leaks,  Distinction  between        . .         . .       63 

caused  mainly  by  a-Eays       . .         . .         . .         • .         •  •       75- 

of  Gases  7,14,  38  et  seq. 

—  by  Extreme  Ultra- Violet  Light 14 

Eadiant  Ions      . .         . .         . .         . .         . .       47 

Uranium  Rays    . .         . .         . .         . .          . .       H 

Proportional  to  Absorption  of  Eays         . .  38,  58 


INDEX  TO   CONTENTS.  203- 

PAGE 

lonisation  of  Gases,  Proportional  to  Density  of  Gas  . .         . .       59 

Theory  of  Gaseous  Conduction        37  et  seq. 

Ions,  a  and  /3  Particles  consist  of          46 

and  Emanations,  Distinction  between     . .         . .         . .         . .       39 

Coefficients  of  Diffusion  of  43 

Critical  Velocity  of 47 

Diminution  of  Electric  Field  by  Convection  of 48 

Mass  of          45,  53 

of  Gaseous  Conduction 39 

Action  of  Electric  Field  on       . .         . .       39 

Produced  by  Zinc  under  Ultra-Violet  Light      . .         . .         . .       52 

Direct  Determination 

of  e  for     . .         . .       53 

Value  of  ejm  for     . .       52 

Kadiant         46 

Recombination  of 41,  42 

Two  Classes  of  Gaseous 46,  73 

Velocity  in  Electric  Field 43 

Irregularity  of  Internal  Atomic  Motion  . .  . .        179,  180 

Jackson,  Herbert  :  Focus  Tube 7 

Joachimsthal  Pitchblende          16,21 

Johanngeorgenstadt  Pitchblende          16, 21 

Kaufmann  :  Conservation  of  Mass        180 

Increased  Mass  of  fast-moving  Corpuscles  from  Radium,  54,  74 

Kelvin,  Lord  :  Age  of  the  Earth  183 

Mechanical  Theory  of  Light     . ,         4 

Kestel :  Maintenance  of  Planetary  Motion 184 

Kinetic  Energy  of  a  and  /3  Particles 46,  81,  123,  163 

Radiant  Ion  . .         . .         . .         . .         . .         . .       46 

Theory  of  Gases 179,  187 

Kunzite 58,  150 

X,  the  Radio-Active  Constant 92,  113,  116,  125,  168 

Langevin  :  Coefficients  of  Recombinations  of  the  Ions         . .         . .       42 

Laplace  :  Nebular  Hypothesis 185 

Last  Stage  of  Disintegration,  /3  and  7-Rays  Produced  in      . .        135,  146 

Latent  Energy  of  Atomic  Structure 34,  122,  170 

Law  of  Conservation  of  Mass     . .         . .         . .         . .         . .        164,  180 

Equivalence  of  Electric  Charges         . .         . .         . .        171,  180 

Radio- Active  Change 91,  114,  122,  176 

Theimodynamics,  The  Second  172,187 

Lead,  Absorption  of  a,  £  and  7-Rays  by          76 

Lecture  Experiment  illustrating  the  Condensation  of  the  Radium 

Emanation     ..         ..          ..         ..         ..         ..         ..         ••     150 

Lenard  :   New  Kinds  of  Radiations       . .         . .         . .         •  •         •  •         2 

Penetrating  Cathode  Rays 9 

Value  of  v  and  elm  for  Cathode  Rays         51 

Life,  Actual  . .  125,  177,  180 

Average         125,147,177 

of  Radium  Atom ..     168  et  seq. 

Uranium  and  Thorium  . .         . .         . .         •  •         •  •  •  •     186 

Light,  Corpuscular  Theory  of 2,  5,  14 

Due  to  Electromagnetic  Oscillations   ..         •«         ••  ••         ^ 

Electromagnetic  Theory  of       . .         . .         •  •         •  •  3,  7,  14 

lonisation  by  extreme  Ultra- Violet 14,  52 


204  RADIO-ACTIVITY. 

PAGE 

Light,    Negative  Ions  expelled  by   Zinc   under  the   Influence   of 

Ultra- Violet 52 

Pressure,  Effect  of,  on  Planetary  Motion       . .          . .          . .  184 

Transverse  Propagation  of        . .          . .          . .          . .          . .  3 

Undulatory  Theory  of 3,  5 

Limited  Range  of  Scientific  Generalisations 187 

Limits  of  Applicability  of  Gold-Leaf  Electroscope     . .          . .  61,  C3 

with  Eespect  to  Past  and  Future  Time  extended      . .          . .  189 

Lives,  Table  of  Average 147 

Lodge  :  Calculation  of  Electrical  Mass  at  Varying  Speeds  . .          . .  54 

Electronic  Kadiation  from  Atom         179 

Lunar  Month,  Connection  between  Terrestrial  Day  and       . .         .-.  184 

McClelland :  Charge  carried  by  Radium  Emanation  . .         . .     141 

Magnesium  Platino-Cyanide,  Non- fluorescence  under  Radium-Kays.       58 
Magnetic  Deviation  of  a-Ray      . .          . .  13,  67,  77  et  seq. 


/3-Ray     . . 
Cathode-Ray 
Radiant  Ion 


Maintenance  of  Planetary  Motion 
Radium  and  Polonium 


Solar  Radiation 


13,  67,  70  et  seq. 
10,  50,  67,  70 
..       49 
,.    '     ..     184 
.          ..     171 
184 


Marckwald :  Amount  of  Polonium  in  Pitchblende      . .  29,  73 

Polonium '29 

Radio-Tellurium I.:    ....       29 

Mass,  an  Atomic  Property          . .          . .          . .          . .          . .     ~~  -v*       20 

Conservation  of 164,180 

Electrical 54,75,80 

of  a  Particle  . .         13,  79,  80 

a  Particles  expelled  per  gramme  of  Radium  per  Hour    . .     168 

/3  Particle          72,  74,  180 

Corpuscle          53,  54,  80,  180 

the  Ions  45 


Material  Nature  of  Imparted  Activity 105 

Properties  contrasted  with  Radio-Active  Properties,  26,  122,  150 

of  Radium  Emanation    ..          ..          ..     149  et  seq. 

Matter,  Electronic  Theory  of 55 

Evolution  of         123,  125,  126,  140,  189 

New  Methods  of  detecting        19,  82,  109,  139 

Maximum  lonisation  with  Thick  Layers  of  Gas        . .         . .         . .       59 

Measurement  of  Radio -Activity 37, 57  et  seq. 

Metabolon 123 

Metabolons,  Radium  and  Polonium  Atoms  are          172 

Methods  of  Measuring  Radio- Activity  . .         . .          . .          . .       57  et  seq. 

Meyer  and  Von  Schweidler  :  Deviation  of  /3-Ray  by  Magnet  . .       70 

Mica  impervious  to  Thorium  Emanation         . .         . .         . .          . .     100 

Minerals,  Age  of  Radio-Active 186 

Maintenance  of  Intensely  Active  Elements  in      . .         145,  171 

Presence  of  Helium  in  Radio-Active  ..          ..         117,163 

Radio -Activity  of        16,116,142,145,172 

Molecular  contrasted  with  Radio-Active  Change         . .  107,  122,  166 

Forces,  Radio-Activity  outside  the  Range  of        . .          93,  122 

Molecules,  Number  of,  in  a  cubic  centimetre  of  Gas  . .          . .          . .       45 

Monazite,  Radio-Activity  of        . .         . .          . .         . .          . .          16,  163 

Mono-molecular  Chemical  Reaction      ..          ..          ..          ..         114,176 

Motion,  Internal  Atomic 178,  179 

Irregularity  of  Internal  Atomic  . .         . .          . .         179,  180 


INDEX  TO   CONTENTS.  205 

Motion,  Maintenence  of  Planetary         ..         ..         ..         ..         ..     184 

Mutual  Action  between  Kadio-Active  Systems  excluded       . .         . .     179 

Natural  lonisation  of  the  Atmosphere  ..         ..         ,„ 3a 

Nature  of  Eadio- Activity  . .          . .         . .         . .     109  et  seq. 

the  Atom          171,  176 

Nebular  Hypothesis         . .         . .         . .         . .         . .         . .         . .     185 

Negative  Charge  carried  by  /3-Rays  of  Kadium  . .         27,  72,  74,  181 

Ions,  Supersaturation  necessary  for  the  Condensation  of 

Water- Vapour  on 45 

Negatively-charged  Particles,  £-Rays  are         13,  72 

Neutrality,  Electrical        J 182 

New  Amount  of  Thorium  X  produced  from  Thorium  . .         . .       88- 

—  Elements  in  Pitchblende 17,  82,  173 

—  Experimental  Methods  of  Detecting  Matter       . .       19,  82,  109,  139 

Newton  :  Action  at  a  Distance 2 

Corpuscular  Theory  of  Light  3,  14 

Niobite,  Radio-Activity  of  16 

Nitrogen  Spectrum  in  Fluorescent  Light  of  Kadium. .         . .        160,  164 
Non-separable  Activity  consists  only  of  o-Rays     88,  95,  96,  113,  133,  137 


Explanation  of.. 
-  of  Radium 
—  Thorium 


Uranium 


112- 
96,  133,  137 
88,  96,  112 
95,  96,  113 


Number  of  a  Particles  produced  at  each  Disintegration       . .        112,  169 

Atoms  changing  measured  by  Radio-Activity        . .         . .     112 

Emanation  Atoms  produced  from  each  Atom  of  Radium      169' 

Ions,  Determination  of,  by  Condensation  of  Water- Vapour      45 

Molecules  in  a  cubic  centimetre  of  Gas 45- 

Numerical  Relations  between  Atomic  Weights          . .         . .         . .     164 

Occlusion  of  Helium  and  the  Emanations  by  Solids. .         . .        117,  158 

Opacity  of  Matter 9,14 

Orangeite,  Radio-Activity  of 1& 

Oscillations,  Electromagnetic     ...        4 

Owens :  Radio-Activity  of  Thorium       . .         . .          . .         . .         . .       99 

Oxidations  effected  by  Radium  Rays 129 

Ozone,  Formation  of,  by  Action  of  Radium  Rays       . .         . .        128,  129- 

Paper,  Permeability  of,  to  Thorium  Emanation         100- 

Parallelism  between  £  and  y  Radiation  69 

Parent  Elements 116,  117, 124 

Reconstruction  of       . .         . .         . .         . .         . .     186- 

Particle,    Charge  carried  by  a 13,  46,  80,  181,  182 

|3 13,  27,  46,  72,  73,  74,  181 

Mass  of  a          13,  79,  80- 

/3  72,  74,  180 


Radiant 5, 46 

Velocity  of  a 13,  79,  80 

B  . .    72,  74,  180' 


Peculiarities  of  the  Radio- Activities  of  different  Radio-Elements,    21,  31, 

32,  142  et  seq. 

Penetrating  Power  of  a-Rays . .  12,  65,  75,  80- 

_ S-Rays  ..         ..     12,27,32,66,70 

1 Q     Oft     fift. 

-y. Rays       ..          ..          ..          ..          ••      10,^0,0* 

Kadiation  from  Imparted  Activity      . .          75,  105 
Three  Types  of  Radium  Rays 27 


206  HA  DIG- A  CTI VITY. 

Penetrating  Power  of  X-Eays  and  Cathode  Rays        8, 9 

Radiation  of  Uranium  a  Measure  of  the  Quantity  of 

Uranium  X          ..         ..         ..         ..         ..          75,105 

X-Rays,  Normal  lonisation  of  Heavy  Vapours  by  69 

Periodic  Law         125,  187 

Permanent  Radio-Elements 115,  173 

Phases  of  Atomic  Property         125 

Phosphorescence  (see  Fluorescence) 

Phosphorus,  Action  of  Radium  Rays  on          -.-.123 

Photographic  Action  mainly  caused  by  0-Rays  70 

of  Radium  Rays 18 

Thorium  Rays 13,70 

Uranium  Rays. .         . .    11,  13,  15,  18,  57,  70,  85 

X-Rays 7 


Method  of  Measuring  Radio-Activity 57, 79 

Physical  Meaning  of  Decay  of  Activity  . .          . .         ..          92,  113 

Radio- Active  Constant 114 

Physiological  Effects  of  Radium  Rays  . .         . .         . .         . ,         . .     129 

Pitchblende,  Age  of          186 

Amount  of  Polonium  in 29,  30,  173 

• Radium  in       ..         ..         ..         ..18,21,173 

Radio-Activity  of 16,21,82,173 

Platinocyanides,  Fluorescence  of  27,58 

Poincare  :  Connection  between  X-Rays  and  Fluorescence    . .          . .       10 

Polar  Flattening  in  Reference  to  Age  of  Earth          184 

Polonium,  Amount  of,  in  Pitchblende 29,  30,  173 

Connection  with  Radium 140,  146,  173,  174 

-  considered  as  a  Transition-Form 173 

Discovery  of     . .         . .          . .          . .          . .          . .          . .       17 

-  Identity  with  Radio-Tellurium        29,  30 

—  Maintenance  of  ..          ..         ,\         ..          ..          ..     171 

-  Methods  of  Separation          22,  28,  29 

-  Peculiarity  of  Radio-Activity  of 29,  32,  175 

-  Radiation  of 29,  65,  76,  136 

-  Rate  of  Decay  of  Activity  of 29,30,113 

Positive  Charge  carried  by  Matter  causing  Imparted  Activity        106,  181 

-    Residue  of  Atom 181 

Charges,  Simultaneous  Production  of  Two     . .         . .         . .     180 

Ions,  a-Rays  are 46,  182 

Supersaturation  necessary  for  the   Condensation   of 

Water  Vapour  on          45 

Rays          53,  182 

Precautions  against  Electrostatic  Disturbance  . .          . .          . .       61 

in  using  Gold-Leaf  Electroscopes. .         .,         . .         . .       63 

Pressure,  Effect  of,  on  Movement  of  Ions        . .          . .          . .          . .       46 

exerted  by  Thorium  Emanation 156 

Flow  of  Crystalline  Rocks  under     . .          . .          . .          . .     184 

—  of  Light,  Effect  of  tn  Cosmical  Processes 184 

Principles  of  Radio-Active  Analysis      . .         . .          . ,         . .          . .     144 

Probabilities,  Law  of,  in  Radio-Active  Change          115 

Production  of  New  Matter  in  Radio-Activity  . .         .  .       89,  94,  103,  110 

Thorium  X,  Constant  Rate  of 93 

Products,  Ultimate          ..        116,158 

Quadrant  Electrometer 59,61 

Quantity  of  Polonium  in  Pitchblende 29,  30,  173 

~ Radium  in  Pitchblende 18,  21,  173 


INDEX  TO   CONTENTS.  207 

Quantity  of  Transition-Form  in  Mineral        P172 

Forms  infinitesimal,    28,83,87,103,106,113,128, 

156,  162 
Quartz,  Use  of,  as  Insulator ,f         ..         ..62,74 

Eadiant  Ions         46 

Determination  of  v  and  e/m  for          ..         ..         ..       50 

Deviation  by  Electric  and  Magnetic  Field    . .         . .       49 

their  Power  of  Ionising  Gases ..         . .         ..         ..       47 

Matter 5,46 

Eadiation,  Corpuscular 5,14,70,77 

Definition  of  a  2 

of  Air  above  Thorium  Compounds  . .         . .         . .     100 

Polonium 29,  65,  76,  136 

Radio-Active  Substances  not  affected  in  Character  by 

Decay  and  Kecovery  110 

Eadium 32,  65  et  seq.,  135,  146 

Thorium 32,  65  et  seq.,  87,  135,  146 

-  Uranium 12,  32,  65  et  seq.,  85,  95,  146 

Phenomena I  et  seq. 

112 


Two  Main  Classes  of 

5,  14,  78 

.      3,  5,  8,  9.  14,  76 

.     21,  33,  142  et  seq. 

Chang0                3fi;  «9,  105,  107 

111  et 

seq.,  146,  158  et  seq. 

Controlling  Influence  in 

176 

Bearing  of,  on  Problem  of  Chemical  Valency  182 

Effect  of  Conditions  upon  . .  . .  93,  103,  134 

-  Energy  of 94,  128,  163  et  seq.,  183 

Equivalence  of  Electric  Charges  produced  in  181 

Law  of 146,  158  et  seq. 

of  Eadium,  Limits  of  Knowledge  of    ..         ..     163 


Constant 92,113,116,125,168 

Constants,  Table  of  the 146 

Elements 15  et  seq. 

similarity  to  Inactive  Elements  in  Chemical 

Properties 26,  126,  170 

Energy,  Source  of  . .  33,  94  121,  167,  182  et  seq. 


Equilibrium 

Induction,  Hypothesis  of 

Measurement 

Phenomena,  Complexity  of 


93,  94,  115,  124 
84,  87,  89,  94,  132 
57  et  seq. 
73,  117,  123,  138 


Properties  contrasted  with  Material  Properties,  26, 122, 150 
of  Radium 27, 127  et  seq. 


Eadio- Activity,  a  Means  of  Qualitative  and  Quantitative  Chemical 

Analysis          20,  21,  33,  142  et  seq. 

a  Measure  of  the  Number  of  Atoms  changing          . .     11^ 

a  New  Weapon  in  the  search  for  New  Elements         19,  82 

a  Property  of  the  Heaviest  Elements 24 

an  Atomic  Property        17,  20,  81,  82,  122 

Bearing  of,  on  Biology I8?'  !89 

Conservation  of  ..         ..         ..         •'         ••        Ho,  138 

Defined  as  the   Simultaneous   Occurrence  of  Two 

Events  ..         ..         ...     •  •         ••         ••     «0 

^_^__  Discovery  of         •  •         •  •         •  •         •  •         •  •         ~*    • 

Impossibility  of  affecting,  by  Artificial  Means,  20,93,103,124 

of  Common  Materials     ..         ..         ••         ••         ••       ^ 


208  RADIO-ACTIVITY. 

PAGE 

Radio- Activity  of  Thorium  X,  Connection  of,  with  Emanating  Power,   110 

Ultimate  Cause  of          55,179 

Radio-Lead 33 

Radio-Tellurium  and  Polonium ..         ..         29,30 

Radium,  a-Rays  of. .    27,  71,  75,  77  et  seq.,  130,  133,  136  et  seq.,  146,  163, 

166,  182 

Analysis  of  the  Radio- Activity  of 137 

a  slow-changing  Transition-Form    . .         •  •         . .          . .     172 

Atom,  Internal  Energy  of  . .          . .          . .         . .     169 

Atomic  Weight  of       ..         ..         ..         ..         ..  23,25 

Average  Life  of  . .         . .         . .         168 

/3-Rays  of          27,  70  et  seq.,  135,  137,  146 

Chemical  Effects  of  the  Rays  of 128 

Discovery  of  . .         . .          . .          . .          . .          . .       17 

Emanation,  Absorption  of,  by  Celluloid 130 

Chemical  Nature  of 130 

Condensation  of  . .          . .   •      *.'       130,  150  et  seq. 

—  Diffusion  Coefficient  of  . .  .         . .         . .     160 

—  Discovery  of        . .          . .          . .          . .         . .     130 

Effect  of  Temperature  on        . .         . .         . .     130 

Heat  Emission  of  166 

—  present  in  Soil  and  Tap-Water  .,          19,  158 

Production  of  Helium  from     . .          . .          . .     160 

• — Polonium  from . .  174 


Radiation  of        136 

Emanation,  Rate  of  Decay  of  133,  134 

Total  Energy  Emitted  by         167 

Energy  emitted  by . .         . .  27,  128,  165 

Explosion  of  SeaLed  Tubes  containing       74 

Flame  Coloration  of    . .          . .          . .          . .          . .          . .       23 

Spectrum  of 26 


7-Rays  of          . ;         . .      13,  28, 

Heat-Evolution  of 27,  165 

High  Velocity  of  Corpuscles  expelled  from  . .  54,  74 

Imparted  Activity  of 33,  75,  132,  138,  142,  146 

Maintenance  of  . .         . .         . .         . .         . .         . .     171 

Method  of  Extraction  of        17,  21,  22 

Non-de viability  of  easily-absorbed  Rays  of  . .  70 

Non-separable  Activity  of     . .         . .  .         . .  96,  133,  137 


Normal  Chemical  Character  of 
Physiological  Effects  of  the  Rays  of 
Position  in  Radio-Activity  occupied  by 
Production  of  Helium  from  . 


_ Polonium  from 

—  Quantity  of,  in  Pitchblende 

—  Radiations  of  (General  Description) . . 

—  Radio- Active  Constant  of 


26,  170 
. .     129 
..     127 
117, 158 
. .     174 
. .    18,  21,  173 
. .    27,  66,  135 
. .     168 
27,  127  et  seq. 
94,  168 
133,134 
Relative  Activity  of,  in  Terms  of  Uranium  . .  18,  24 

Salts,  Increase  of  Activity  of,  with  time 84 

Self -Coloration  of  Salts  of 23 

Self-Electrification  of  28,  73,  74,  181 

Source  of          . .         . .         . .         . .         . .         . .         . .       21 

Summary  of  Radio-Active  Changes  in        137 

Test  for  the  Presence  of        142 


Rate  of  Change  of 


Properties  of 


Recovery  of  Activity  of 


INDEX   TO   CONTENTS.  209 

Kadium,  View  that  Uranium  produces  ..         ..         ..         ..     175 

widely  diffused  in  Nature 19 

Ramsay,  Sir  William  :  Methods  of  Gas  Manipulation          . .         . .     117 

Presence  of  Helium  in  Minerals      . .          . .         . .         . .     117 

Ramsay,  Sir  William,  and  Soddy :  Oxidising  Action  of  the  Radium 

Emanation     . .         . .         . .         . .         . .         . .         . .     129 

Production  of  Helium  from  Radium  . .         . .         . .     158 

—  Volume  of  the  Radium  Emanation  . .         . .         . .         . .     161 

Rapid  Internal  Motion  of  Atom  . .         . .         . .         . .         . .     178 

Rare-Earth  Elements 177,  178 

-  Gases  of  the  Atmosphere . .         . .         . .         . .         . .         . .     117 

Rarefied  Gases,  Discharge  of  Electricity  in 46  et  seq. 

Rate  of  Radio-Active  Change  proportional  to  Amount  unchanged  . .     114 

Rayless  Changes 139,  141 

Rays  (see  a-Rays,  0-Rays,  7-Rays,  Cathode  Rays,  X-Rays). 

Recombinations  of  Ions 41,  42 

Reconstruction  of  Matter  . .         . .         . .         . .       185,  186  et.  seq. 

Recovery  of  Activity  of  Radium  133,  134 

Thorium          87,  88,  90,  92,  117 

Uranium          85,  94 

Emanating  Power  of  Thorium 87 

Regeneration  of  Emanating  Power  in  De-emanated  Compounds    . .     120 

Emanation  after  Removal 132 

Thorium  X  after  Removal 88,  93 

Relation  between  Current  and  Voltage  in  Ionised  Gas         . .  40,  47 

Reproduction  of  Thorium  X        88 

Thorium  X,  Constant  Rate  of          93 

Residue  of  Atoms,  Positive  Charge  carried  by          181 

Rontgen  :  Discovery  of  X-Rays 6 

Researches  of 

Runge  and  Precht :  Spectrum  of  Radium       . .         . .         . .  25,  26 

Rutherford  :  Absorption  of  a-Rays  by  Matter . .         . .         . .         . .       77 

Character  of  a-Rays 77 

— Radiation  from  Thorium  Emanation          . .     101 

—  Charge  carried  by  Thorium  Emanation     ..         ..        101,141 

Complexity  of  Radiation  from  Uranium    . .         . .         . .       65 

-  Connection  between  Emanation  and  Imparted  Activity ..     Ill 

Helium  and  a  Particle          . .         . .     163 

Decay  of  Activity  of  Thorium  Emanation . .         . .         . .     102 

—  De-emanation  of  Thorium  Compounds 1 19 

Deviation  of  a-Ray  by  Magnetic  Field       . .  .  67,  77 


Discovery  of  Imparted  Activity  of  Thorium 


7-Rays  of  Thorium  and  Uranium 

the  Thorium  Emanation 

Effect  of  Electric  Field  on  Imparted  Activity 

Electrical  Method  of  Measuring  Radio-Activity 


Explanatian  of  7-Rays 
Imparted  Activity  of  Radium 
Importance  of  a-Rays 

Interpretation  of  Curve  of  Recovery  of  Activity 
lonisation  and  Absorption  proportional  to  Pressure  of  Gas      58 
produced  by  Uranium     ..         ..         ••  37,58 


32,  105 
..       67 
32,99 
106,  181 
..       58 
..       69 
..     139 
..       75 
93 


__  Mass  of  a  Particles  expelled  per  gramme  of  Radium  per  hour     168 
_  .  Non-Refrangihility  of  Uranium  Radiation  .  .         •  •       12 

__  Parallelism  between  /S  and  7-Rays  ......          69,  lc 

Penetrating  Power  of  7-Rays 


-  Properties  of  Imparted  Activity  of  Thorium 


Badio-Activity. 


210  RADIOACTIVITY. 

PAGE 

Rutherford :  Rate  of  Movement  of  Ions  in  Electric  Field    . .  . .       42 

—  Theory  of  Radio-Active  Induction  . .          . .          . .  . .       84 

Rutherford  and  Barnes :  Heat-Emission  of  Radium  Emanation    . .     166 
Rutherford  and  Miss  Brooks  :  Characteristics  of  Radiations  of  the 

various  Radio-Elements  . .          . .         . .          . .  31,  75 

Complexity  of  Changes  causing  Imparted  Activity  . .     138 

—  Diffusion  Coefficient  of  the  Radium  Emanation    . .  . .     157 
Rutherford  and  Grier :  /3  Radiation  of  Thorium        . .         . .  66,  70 

-  #  Radiation  of  Uranium  X   . .         . .          85 

Rutherford  and  McClung :  Energy  emitted  by  Uranium      . .          . .     165 
Rutherford  and  Soddy  :  Amount  of  Emanation  in  Non-Emanating 

Compounds  ..         131 

—  /3  Radiation  of  Radium          136 

Condensation  of  the  Radio-Active  Emanations    104,  152  et  seq. 

Connection  between  Kadio-Activity  and  Emanating  Power 

of  Thorium  X        110 

Decay  of  Activity  of  Radium  Emanation  . .         . .         . .  133 

Thorium  X 86 

Uranium  X 94 

Discovery  of  Thorium  X       . .         ......         . .  86 

Metabolon         123 

Nature  of  the  Thorium  Emanation. .         .  *         . .          . .  103 

Occlusion  of  the  Emanations  by  Solids 117 

Prediction  of  Helium  as  an  Ultimate  Product       . .         . .  117 

Theory  of  Atomic  Disintegration 82,  109 


Samarskite,  Radio-Activity  of 16 

Same  Change  produces  Rays  and  New  Matter  112 

Saturation  Current  . . 39,  42,  58,  60,  63 

Scheelite,  Fluorescence  of          58 

Schmidt :  Discovery  of  the  Radio-Activity  of  Thorium       . .         . .       15 

Scintillations  of  Zinc  Sulphide 27,  80 

Second    Law  of    Thermodynamics,   Applicability    to   Sub-atomic 

Change  187 

Secondary  Emanation  of  Actinium      ..         ..         ..         ..         ..     142 

Selection,  Law  of  Natural,  applied  to  Matter 126 

Self-Electrification  of  Radium 28,  73,  74,  181 

Separation  of  the  Radio-Active  Constituents  of  Pitchblende  . .       22 

Radium       ..         ..137 

Thorium      ..          ..      118 

Uranium      ..  83,95 

Shape  of  Earth  defined  by  Period  of  Rotation  184 

Shrinkage,  Solar 185 

Similarity  of  Active  and  Inactive  Elements  in  Chemical  Properties,  26, 

126,  170 

all  Cases  of  Radio-Activity         28,  80 

Simultaneous  Production  of  Two  Positive  Charges 181 

Soddy  (see  Ramsay) 

(see  Rutherford) : 

Effect  of  a-Rays  of  Uranium  on  Photographic  Plate  57,  85 

Persistence  of  Emanating  Power  in  Absence  of  Air  . .     104 

Radio-Activity  of  Uranium       . .          . .          . .          . .          . .       85 

View  that  Polonium  is  produced  from  Radium        . .          . .     175 

Radium  is  produced  from  Uranium         . .          . .     175 


4 'Soft  Tube' 

Solar  Radiation,  Maintenance  of          184 

Shrinkage 185 


INDEX   TO   CONTENTS.  211 

PAGE 

Solar  System,  Idea  of,  applied  to  Atoms         177 

Solutions,  Emanating  Power  of ..         ..     119 

Sorting-Demon,  Clerk-Maxwell's  187 

Source  of  Energy  necessary  for  Atomic  Reconstruction       . .         . .     187 
-   Kadio-Active  Energy. .         . .  33,  94,  121,  167,  182  et  seq. 

Eadium  21 

Spark  Discharge,  Explanation  of          47,  48 

Sparteite,  Fluorescence  of  . .         . .         . .         . .         . .         . .       58 

Specific  Activity  of  Thorium . .         , .  91,  97 

Nature  of  Thorium  X    . .         89 

Spectrum  of  Radium 17,  24,  25 

Reaction,   Near  Approach  to  an  Atomic  Property          . .       20 

Sensitiveness  of,  compared  to  Radio- Activity  ..       19 

Spinthariscope 27, 80 

Spodumene,  Fluorescence  of 58 

Spontaneity  of  Energy  Emission  in  Radio-Activity 34 

Spontaneous  Expulsion  of  Radiant  Ions          . .         . .         . .    47,  81,  123 

lonisation  of  the  Atmosphere    . .          . .         . .  .       38 


Transmutation,  Radio-Activity  due  to  . . 


Stable  and  Unstable  Elements  contrasted 
Stability  of  the  Elements,  Reason  of 


94 
124 
126 

Stokes,  Sir  George :  Cause  of  the  X-Rays 

Striae  49 

Structure,  Atomic  54,  93,  122,  125,  118  et  seq. 

Strutt :  Anomalous  Cases  of  lonisation  in  Gases      . .         . .  59,  68 

lonisation  of  Gases  proportional  to  Density  . .         . .       59 

Limit  of  Detection  of  Radio-Activity 19 

Negative  Charge  carried  by  /3-Rays  of  Radium       . .         . .       73 

Occurrence  of  Helium  in  Minerals 163 

Sub-atomic  Changes  unaccompanied  by  Radiation    . .         . .         . .     139 

Subsequent  Changes,  Disturbing  Effects  due  to         . .  117,  120,  134 

Successive  Changes,  Tables  of 107,  146 

Summary  of  Radio-Active  Changes  in  Radium          137 

Supersaturation  necessary  to  cause  Condensation 44 

Survival  of  the  Fittest,  Law  of,  applied  to  Matter 125 

Unfit  by  Continuous  Reproduction 125 

Table  of  Coefficients  of  Diffusion  of  Ions 43 

Ionic  Velocities  . .          . .         . .         . .         . .         •  •       43 

the  Analysis   of    the  Radio- Activities  of  Uranium  and 

Thorium  . .         . .          . .         . .         . .         . .       88 

Characteristics  of  the  Radio-Elements         . .  30, 31 

146 
147 
62 
16 

24,25 
107 


Disintegration  Series  of  the  Radio-Elements 

Radio-Active  Constants  and  Average  Lives  . 


Radio-Activity  of  Minerals 


Uranium  Compounds 


Spectrum  Lines  of  Radium 
Successive  Changes  in  Thorium 


Tait :  Age  of  the  Earth 182,  1£ 

Tap-Water,  Presence  of  Radium  Emanation  in         . .         .  •          19.  I58 
Tellurium,  Presence  of,  in  Pitchblende  . .         . .         •  •         •  •       ^0 

Temperature,  Effect  of,  on  Emanating  Power  . .         •  •        120,  131 

Radium  Emanation        . .         134, 150  et  seq. 

Thorium  Emanation      . .  103,  104,  152 

Independence  of  Radio-Active  Change  on,  93,  103, 134, 177 

of  Volatilisation  and  Condensation  of  the  Radium 

and  Thorium  Emanation     . .         . .         •  •         •  •     1<*3 


212  RADIO- ACTIVITY. 

PAGE 

Temporary  Kadio-Activity,  Cases  of 85,  92,  103,  113 

Ten-Million  Year  Estimate  of  the  Age  of  the  Earth 183 

Terrestrial  Day,  Increase  of 184 

Thermodynamics,  Second  Law  of         187 

Thick  and  Thin  Layers  of  Thorium  Compounds  contrasted  99,  100 

Thompson,  S.  P. :  Transparency  of  Ebonite 9 

Thomson,  J.  J. :  Corpuscular  Theory  of  Electricity 53 

Determination  of  Charge  carried  by  an  Ion          . .         . .       45 

Negative    Ion     pro- 
duced by  Ultra-Violet  Light  on  Zinc        53 

Ratio  elm  for  Negative  Ion       . .          . .  50,  72 

: — Velocity  of  Negative  Ion  . .          . .       50 

Equivalence  of  Positive  and  Negative  Charges  produced 

during  Disintegration  of  Eadium  . .          . .          . .     181 

Increase  of  Electrical  Mass  with  Velocity 54 

lonisation  Theory  of  Gaseous  Conduction  . .       37  et  scq. 

Presence  of  Eadium  Emanation  in  Common  Materials,  19,  158 

Thorite,  Eadio-Activity  of          16 

Thorium,  a  Eadiation  of 15,  66,  75,  88,  95,  146 

Analysis  of  the  Eadio-Activity  of    . .         . .  82,  86,  88,  106  118 

Apparent  Variability  of  the  Eadio-Activity  of  . .       99 

Average  Life  of 186 

j8  Eadiation  of 15,  66,  70,  88,  135,  146 

Curve  of  the  Eecovery  of  Activity  of          . .         . .          90,  118 

Discovery  of  Eadio-Activity  of         . .         . .         . .         . .       15 

Emanation 32,  86,  99  et  geq.,  143,  152  et  seq. 

Chemical  Nature  of        103 

Condensation  of  . .         . .         . .        104, 152  et  seq. 

Effect  of  Temperature  on         . .  108,  104,  152 

Emanation,  Eadiation  of 101 

Eate  of  Decay  of  Activity  of    . .         . .         101-104 

from  Gadolinite,  Inactivity  of          96,  97 

Imparted  Activity  of  . .         33,  75,  101,  105,  120,  138,  143,  146 

Test  for  the  Presence  of        143 

Thorium  X,  a  Specific  Type  of  Matter 86,  89,  100 

Discovery  of  . .          . .          . .          . .         . .          . .       86 

Infinitesimal  Amount  of 87 

Eadiations  of  135 

. accompanying  the  Production  of  Emana- 
tion ..         ..         112 

Eate  of  Decay  of  Activity  of         . .         . .         .  90 

..       86 
88,  104,  110 
..     186 
183 


Separation  from  Thorium . 

the  Source  of  the  Emanation       . .         . .       87 

Thousand-Million  Year  Estimate  of  the  Age  of  the  Earth   . 
Tidal  Eetardation  in  reference  to  the  Age  of  the  Earth 


Time,  Consistency  of  Laws  of  Nature  with  Eeference  to  Past  and 

Future 189 

Effect  of,  on  Proportion  of  Thorium  Emanation  condensing  155 

-  of  Past  Existence,  Average  Life  Independent  of        . .          . .  178 

Tin,  Absorption  of  a  and  /3-Eays  by 76 

Townsend  :  Critical  Ionic  Velocity       . .          . .         . .         . .          . .  43 

Diffusion  Coefficients  of  Ions  in  Gases      . .         . .         •  •  47 

Transmutation  of  Atoms            93 

of  the  Eadium  Emanation  into  Helium        . .          . .  160 

Eadio-Activity  due  to  a  spontaneous            ..         ..  94 


Transparency  and  Electrical  Properties  . .  . .  9,  14 

Triple  Change,  Imparted  Activity  of  Eadium  due  to  a         . .        138,  139 


INDEX  TO   CONTENTS.  213 

Ultimate  Cause  of  Radio- Activity         55*179 

Products  of  Radio- Active  Change 116,  158 

Ultra- Violet  Light,  lonisation  of  Gases  by 14,  52 

Negative  Ions  expelled  from  Zinc  under  . .       52 

Undulatory  Theory  of  Light 3 

Universe  a  Conservative  System  189 

Unstable  Elements  124 

Up-building,  Atomic        186 

Uraninites,  Age  of  the 186 

Uranium,  a  Radiation  of  15,  65,  75,  85,  88,  96,  146 

Absence  of  Emanation  from  32,  95,  175 

—  Analysis  of  Radio-Activity  of          83,  88,  95 

-  /3  Radiation  of  . .          . .      15,  57,  66,  70,  85,  94,  135,  146 

—  Compounds,  Radio- Activity  of  the  Various          . .         . .       16 
•  Discovery  of  the  Radio- Activity  of 2,  10 

—  Enfeeblement  of  Radio-Activity  of,  by  Chemical  Processes     84 
Rate  of  Recovery  of  Activity  of        . .         . .         . .         . .       94 

—  Rays,  Complexity  of 65 

Non-refrangibility  of  . .         . .         . .         . .          . .       10 

—  Photographic  Action  of          . .         . .           11,  15,  83,  70 
Similarity  of,  to  X-Rays        11 

—  Relative  Activity  of  Radium  and 18,  24 

Simplicity  of  the  Radio-Active  Change  of 95 

the  Parent  Element  of  Radium  and  Polonium     . .        173,  175 

Uranium  X,  Discovery  of  . .         . .         . .         . .          . .         . .       83 

Methods  of  Separation 83,  84 

Infinitesimal  Amount  of    . .         . .         . .         . .         . .       83 

Radiation  of  85,  95 

Rate  of  Decay  of  Activity  of       . .         . .         . .         . .       94 

Valency,  Bearing  of  Radio-Activity  on  the  Problem  of  Chemical  . .     182 

Vapours,  Absorption  of  X-Rays  by  Heavy        68,  69 

Variability  of  the  Radio- Activity  of  Thorium,  The  Apparent  . .       99 

Velocities,  Ionic 43 

Velocity  of  a  Particle 13,  79,  80 

0  Particle 54,  72,  74,  180 

—  Ions,  The  Critical 47 

—  Positive  Ray 53 

—  Radium  Corpuscle 51,54,180 

66 

-.40,49 
..  153 
..  157 
..  161 

27,  128 
..       44 
43 


Villard  :  Discovery  of  7-Rays  of  Radium 
Viscosity  of  Gases,  Effect  of,  on  Ionic  Velocity 
Volatilisation  of  the  Emanations,  The  Temperatures  of 


—  Imparted  Activity  Matter 


Volume  of  the  Radium  Emanation 

Water,  Decomposition  of,  by  Radium  Rays 
Vapour,  Condensation  of,  on  Ions 


—  Effect  of,  on  Movement  of  Ions 


Wiechert :  Direct  Determination  of  Velocity  of  Cathode-Ray          . .       51 

Wien :  Charge  carried  by  a-Ray 182 

Wilhelmy  :  Law  of  Mono-molecular  Reaction  . .         . .         . .     176 

Willemite,  Fluorescence  of          28,  58,  150 

not  Deteriorated  by  Prolonged  Action  of  the  Radium  Rays,  128 

Wilson,  C.  T.  R.  :  Condensation  of  Water- Vapour  on  Ions  . .         . .       44 

—  Use  of  Gold-Leaf  Electroscope          61 

Wilson,  H.  A. :  Striae . .       49 

Wood :  Screen  Transparent  to  Ultra-Violet  Light 


214 


RADIO-ACTIVITY. 


X-Rays,  Cause  of 

Connection  with  Cathode  Bays 

Fluorescence 

: 7-Eays 

lonisation  of  Heavy  Vapours  by 

—  Produced  by . . 


Nature  of 

Normal  Absorption  of  Penetrating 

of  Eontgen          ..         ....        .:.r "" 

Penetrating  Power  of 
Eadial  Propagation  of 


Xenon,  Amount  of,  in  the  Atmosphere 


PAGE 

. .  8,  69 

8,  10,  69 

..       10 

13,  69 

68,  69 

7,12,37,  43 

8 

..       69 

6,  7,  8,  9,  10 

9 

:  -,,.-    10 

30 


Zeleny  :  Ionic  Velocities 

Zerban  :  Inactivity  of  Thorium  from  Gadolinite 

Zinc,  Negative  Ions  produced  by  Ultra-Violet  Light  on 

-  Sulphide,  Fluorescence  of,  under  Polonium 
Eadium 


Preparation  of  a  Screen  with 
Scintillations  of . . 


...     43 

..       97 

..       52 

27,  58,  80 

..       30 

..     151 

27,  80 


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V  0  3  1993 


MAR  22  1943 


59VMrODSCCikv    NO.    3 '93 


ID 


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